Solipsism (pronounced /ˈsɒlɨpsɪzəm/) is the philosophical idea that only one's own mind is sure to exist. Solipsism is an epistemological or ontological position that knowledge of anything outside one's own specific mind is unjustified. The external world and other minds cannot be known and might not exist. In the history of philosophy, solipsism has served as a skeptical hypothesis.
There are varying degrees of solipsism that parallel the varying degrees of serious skepticism.
Metaphysical solipsism is the "strongest" variety of solipsism. Based on a philosophy of idealism, the metaphysical solipsist maintains that he is the whole of reality and that the external world and other persons are representations of that self having no independent existence. Few, if any, have taken this position beyond hypothetical discussion.
Even physical sight depends on the imagination. If we behold an object, it is not scientific to say, "I see;" but we ought to say, "I imagine to see."
A painter paints a picture upon a piece of wood, and you will then see the picture, but not the wood; but a wet rag may wipe out all that the painter has made. Thus we have been cut out by the hand of God, and He formed us in the three Substances and painted us all over with Life, but death wipes out the picture. Therefore we should not allow ourselves to be seduced by the temptations of life, seeing that they are nothing but illusions, resembling colours which in themselves are neither red, nor yellow, nor green, but merely appear to be so to the eye.
Philosophy informs us that the world is made out of the will of God. If, then, all things are made out of will, it logically follows that the causes of all internal diseases are also originating within the will. All diseases, such as are not caused by any action coming from the outside, are due to a perverted action of the will in man, such as is not in harmony with the laws of Nature or God.
The world of corporeal forms is an external expression of the world of mind. Each thing represents an idea; each star in the sky is a visible symbol of a universal power or principle.
As it is outside of man, so is it inside, and vice versa, for the outside and inside are essentially one thing, one constellation, one influence.
Nature (Heaven) is Man, and Man is Nature; all men are one universal Heaven, and Heaven is only one universal Man.
All the organs of the body, and the body itself, are only form-manifestations of previously and universally existing mental states.
-- The Life of Philippus Theophrastus Bombast of Hohenheim Known by the Name of Paracelsus and the Substance of his Teachings, by Franz Hartmann, M.D.
Epistemological solipsism, or Strong-agnostic solipsism (meaning "knowledge is impossible"), is the variety of idealism according to which only the directly accessible mental contents of the solipsistic philosopher can be known with certainty. Proponents of this view believe it is possible that either an external world exists or that only the self exists, but that it is actually impossible to prove either way.
Methodological solipsism may be a sort of weak agnostic (meaning "missing knowledge") solipsism. It is a consequence of strict epistemological requirements for "Knowledge" (e.g. the requirement that knowledge must be certain). They still entertain the points that any induction is fallible and that we may be brains in vats.
Importantly, they do not intend to conclude that the stronger forms of solipsism are actually true. Methodological solipsists simply emphasize that justifications of an external world must be founded on indisputable facts about their own consciousness. The Methodological solipsist believes that subjective impressions (Empiricism) or innate knowledge (Rationalism) are the sole possible or proper starting point for philosophical construction (Wood, 295). Often methodological solipsism is not held as a belief system, but rather used as a thought experiment to assist skepticism (e.g. Descartes' cartesian skepticism).
Denial of materialistic existence, in itself, does not constitute solipsism.
Possibly the most controversial feature of the solipsistic worldview is the the denial of the existence of other minds. Since qualia, or personal experiences, are private and ineffable, another being's experience can be known only by analogy.
The Law of Analogy in the plan of structure between the trans-Solar systems and the intra-Solar planets, does not necessarily bear upon the finite conditions to which every visible body is subject, in this our plane of being. In Occult Science this law is the first and most important key to Cosmic physics; but it has to be studied in its minutest details and, “to be turned seven times,” before one comes to understand it. Occult philosophy is the only science that can teach it.
Everything in the Universe follows analogy. “As above, so below”; Man is the microcosm of the Universe. That which takes place on the spiritual plane repeats itself on the Cosmic plane. Concretion follows the lines of abstraction; corresponding to the highest must be the lowest; the material to the spiritual.
-- The Secret Doctrine -- The Synthesis of Science, Religion, and Philosophy, by Helena Petrovna Blavatsky
Philosophers try to build knowledge on more than an inference or analogy. The failure of Descartes' epistemological enterprise brought to popularity the idea that all certain knowledge may go no further than "I think; therefore I exist" (cogito ergo sum) without providing any real details about the nature of the "I" that has been proven to exist.
The theory of solipsism also merits close examination because it relates to three widely held philosophical presuppositions, which are themselves fundamental and wide-ranging in importance. These are that:
1. My most certain knowledge is the content of my own mind—my thoughts, experiences, affects, etc.;
2. There is no conceptual or logically necessary link between mental and physical—between, say, the occurrence of certain conscious experience or mental states and the 'possession' and behavioral dispositions of a 'body' of a particular kind (see the brain in a vat); and
3. The experience of a given person is necessarily private to that person.
Solipsism is not a single concept but instead refers to several worldviews whose common element is some form of denial of the existence of a universe independent from the mind of the agent.
Each thing is a state of mind, because the whole world is mind. Each thing is a materialised thought (a "star"), and represents the character of the thought expressed in it.
-- The Life of Philippus Theophrastus Bombast of Hohenheim Known by the Name of Paracelsus and the Substance of his Teachings, by Franz Hartmann, M.D.
Gorgias (of Leontini)
Solipsism is first recorded with the Greek presocratic sophist, Gorgias (c. 483–375 BC) who is quoted by the Roman skeptic Sextus Empiricus as having stated:
• Nothing exists;
• Even if something exists, nothing can be known about it; and
• Even if something could be known about it, knowledge about it can't be communicated to others.
Much of the point of the Sophists was to show that "objective" knowledge was a literal impossibility. (See also comments credited to Protagoras of Abdera).
The foundations of solipsism are in turn the foundations of the view that the individual's understanding of any and all psychological concepts (thinking, willing, perceiving, etc.) is accomplished by making analogy with his or her own mental states; i.e., by abstraction from inner experience. And this view, or some variant of it, has been influential in philosophy since Descartes elevated the search for incontrovertible certainty to the status of the primary goal of epistemology, whilst also elevating epistemology to "first philosophy".
Psychology and psychiatry
Philosophical solipsism as pathological
Solipsism is often introduced (for example "Philosophy made simple", by Popkin and Stroll) as a bankrupt philosophy, or at best bizarre and unlikely. Alternatively, the philosophy is introduced in the context of relating it to pathological psychological conditions.
Solipsism syndrome is a dissociative mental state. It is only incidentally related to philosophical solipsism. Solipsists assert that the lack of ability to prove the existence of other minds does not, in itself, cause the psychiatric condition of detachment from reality. The feeling of detachment from reality is unrelated to the question of whether the common-sense universe exists or not. 
Developmental psychologists commonly believe that infants are solipsist, and that eventually children infer that others have experiences much like theirs and reject solipsism (see Infant metaphysics).
To discuss consequences clearly, an alternative is required: solipsism as opposed to what? Solipsism is opposed to all forms of realism and many forms of idealism (insofar as they claim that there is something outside the idealist's mind, which is itself another mind, or mental in nature). Realism in a minimal sense, that there is an external universe is most likely not observationally distinct from solipsism. The objections to solipsism therefore have a theoretical rather than an empirical thrust.
Solipsists may view their own pro-social behaviors as having a more solid foundation than the incoherent pro-sociality of other philosophies. Indeed, they may be more pro-social because they view other individuals as actually being a part of themselves. Furthermore, the joy and suffering arising from empathy is just as real as the joy and suffering arising from physical sensation. They view their own existence as human beings to be just as speculative as the existence of anyone else as a human being. Epistimological solipsists may argue that these philosophical distinctions are irrelevant since the professed pro-social knowledge of others is an illusion.
The British philosopher Alan Watts wrote extensively about this subject.
Last surviving person
Would the last person left alive be a solipsist? Not necessarily, because for the solipsist, it is not merely the case that they believe that their thoughts, experiences, and emotions are, as a matter of contingent fact, the only ones that can exist. Rather, the solipsist can attach no meaning to the supposition that there could be thoughts, experiences, and emotions other than their own—that events may occur or objects or people exist independently of the solipsist's own experiences. In short, the metaphysical solipsist understands the word "pain" [i.e., someone else's], for example, to mean "one's own pain"—but this word cannot accordingly be construed to apply in any sense other than this exclusively egocentric, non-empathetic one.
Relation to other ideas
Idealism and materialism
One of the most fundamental debates in philosophy concerns the "true" nature of the world—whether it is some ethereal plane of ideas, or a reality of atoms and energy. Materialism posits a separate 'world out there' that can be touched and felt, with the separate individual's physical and mental experiences reducible to the collisions of atoms and the interactions of firing neurons. The only thing that dreams and hallucinations prove are that some neurons can misfire and malfunction, but there is no fundamental reality behind an idea except as a brain-state. Idealists, on the other hand, believe that the mind and its thoughts are the only true things that exist. This doctrine is often called Platonism after its most famous proponent. The material world is ephemeral, but a perfect triangle or "love" is eternal. Religious thinking tends to be some form of idealism, as God usually becomes the highest ideal (such as Neoplatonism). On this scale, solipsism can be classed as idealism, specifically subjective idealism. Thoughts and concepts are all that exist, and furthermore, only 'my' thoughts and consciousness exist. The so-called "reality" is nothing more than an idea that the solipsist has (perhaps unconsciously) created.
There is another option: the belief that both ideals and "reality" exist. Dualists commonly argue that the distinction between the mind (or 'ideas') and matter can be proven by employing Leibniz' principle of the identity of indiscernibles, which states that if two things share all the exact same qualities, then they must be identical, as in indistinguishable from each other and therefore one and the same thing. Dualists then attempt to identify attributes of mind that are lacked by matter (such as privacy or intentionality) or vice versa (such as having a certain temperature or electrical charge). One notable application of the identity of indiscernibles was by René Descartes in his Meditations on First Philosophy. Descartes concluded that he could not doubt the existence of himself (the famous cogito ergo sum argument), but that he could doubt the (separate) existence of his body. From this he inferred that the person Descartes must not be identical to the Descartes body, since one possessed a characteristic that the other did not: namely, it could be known to exist. Solipsism agrees with Descartes in this aspect, and goes further: only things that can be known to exist for sure should be considered to exist. The Descartes body could only exist as an idea in the mind of the person Descartes Descartes and dualism aim to prove the actual existence of reality as opposed to a phantom existence (as well as the existence of God in Descartes' case), using the realm of ideas merely as a starting point, but solipsism usually finds those further arguments unconvincing. The solipsist instead proposes that his/her own unconscious is the author of all seemingly "external" events from "reality".
Philosophy of Schopenhauer
The World as Will and Representation is the central work of Arthur Schopenhauer. Schopenhauer saw the human will as our one window to the world behind the representation, the Kantian thing-in-itself. He believed, therefore, that we could gain knowledge about the thing-in-itself, something Kant said was impossible, since the rest of the relationship between representation and thing-in-itself could be understood by analogy to the relationship between human will and human body.
The idealist philosopher George Berkeley argued that physical objects do not exist independently of the mind that perceives them. An item truly exists only so long as it is observed; otherwise, it is not only meaningless, but simply nonexistent. The observer and the observed are one. Berkeley does attempt to show things can and do exist apart from the human mind and our perception, but only because there is an all-encompassing Mind in which all "ideas" are perceived – in other words, God, who observes all. Solipsism agrees that nothing exists outside of perception, but would argue that Berkeley falls prey to the egocentric predicament – he can only make his own observations, and can't be truly sure that this God or other people exist to observe "reality". The solipsist would say it is better to disregard the unreliable observations of alleged other people and rely upon the immediate certainty of one's own perceptions.
Rationalism is the philosophical position that truth is best discovered by the use of reasoning and logic rather than by the use of the senses (see Plato's theory of Forms). Solipsism, which holds a similar distrust for sense-data, is thus related to rationalism, and in fact may be seen as a form of extreme rationalism.
The theory of solipsism crosses over with the theory of the philosophical zombie in that all other seemingly conscious beings actually lack true consciousness, instead they only display traits of consciousness to the observer, who is the only conscious being there is.
Falsifiability and testability
Solipsism is not a falsifiable hypothesis as described by Karl Popper or Imre Lakatos: there does not seem to be an imaginable disproof. Not even the complete death of the solipsist could falsify his belief in solipsism because he could not analyze that observation.
The method of the typical scientist is materialist: they first assume that the external world exists and can be known. But the scientific method, in the sense of a predict-observe-modify loop, does not require the assumption of an external world. A solipsist may perform a psychological test on themselves, to discern the nature of the reality in their mind (David Deutsch uses this fact to counter-argue below). This investigation may not be proper science, however, since it would not include the co-operative and communitarian aspects of scientific inquiry that normally serve to diminish bias.
Solipsism is a form of logical minimalism. Many people are intuitively unconvinced of the nonexistence of the external world from the basic arguments of solipsism, but a solid proof of its existence is not available at present. The central assertion of solipsism rests on the nonexistence of such a proof, and strong solipsism (as opposed to weak solipsism) asserts that no such proof can be made. In this sense, solipsism is logically related to agnosticism in religion: the distinction between believing you do not know, and believing you could not have known.
However, minimality (or parsimony) is not the only logical virtue. A common misapprehension of Occam's Razor has it that the simpler theory is always the best. In fact, the principle is that the simpler of two theories of equal explanatory power is to be preferred. In other words: additional "entities" can pay their way with enhanced explanatory power. So the realist can claim that, while his world view is more complex, it is more satisfying as an explanation.
While solipsism is not generally compatible with traditional views of God, it is somewhat related to Pantheism, the belief that everything is God and part of God. The difference is usually a matter of focus. The pantheist would tend to identify their self as being a part of everything in reality, which is actually all God beneath the surface. For instance, many ancient Indian philosophies advocate the notion that all matter (and thus humans) is subtly interconnected with not only one's immediate surroundings, but with everything in the universe. They claim that the perception of absolutely-independent beings and things is an illusion that leads to confusion and dissatisfaction -- Samsara. The solipsist, however, would be more likely to put him- or herself in the center, as the only item of reality, with all other beings in reality illusions. It could be said to be another naming dispute; "The Universe" / "God" for the pantheist is "My Unconscious Mind" / "Me" for the solipsist.
Some solipsists believe that some tenets of eastern philosophies are similar to solipsism. Taoism and several interpretations of Buddhism, especially Zen, teach that the distinction between self and universe is arbitrary, merely a habit of perception and an artifact of language. This view identifies the unity of self and universe as the ultimate reality. Zen holds that each individual has 'Buddha Mind': an all-pervading awareness that fills their entire existence, including the 'external' world. This need not imply that one's mind is all that exists, as with solipsism, but rather that the external universe is experienced through the mind of the individual.
Advaita is one of the six most-known Hindu philosophical systems, and literally means "non-duality". Its first great consolidator was Adi Shankaracharya, who continued the work of some of the Upanishadic teachers, and that of his teacher's teacher Gaudapada. By analyzing the three states of experience—waking, dreaming, and deep sleep—he established the singular reality of Brahman, in which Brahman, the universe and Atman, the self are one and the same.
In the Hindu model, Brahman, the god identified with the ultimate all-inclusive reality, plays a game of hide and seek with itself. In this game, called Lila, Brahman plays with individual people, birds, rocks, and other features of the world both separately and together, while forgetting that the game is being played. At the end of each session, Brahman is said to wake up, cease the game, applaud itself, and resume the game all over again. The state of wakefulness and enlightenment is knowing one is simply playing a game; one is simply acting as a human being, having an illusion of being locked within a physical body and separated from the whole of the cosmos.
One who sees everything as nothing but the Self, and the Self in everything one sees, such a seer withdraws from nothing.
For the enlightened, all that exists is nothing but the Self, so how could any suffering or delusion continue for those who know this oneness?
— Ishopanishad: sloka 6, 7
The philosophy of Vedanta, "Aham Brahmasmi" (roughly translated as "I am the Absolute Truth"), could be interpreted as solipsism in one of its primitive senses, as the world is but an illusion in the mind of the observer. However, Advaita Vedanta can be understood to be non-solipsistic when it is recognised that it does not actually deny the existence of a world 'external' to the Self or Atman. Rather, it is asserting that the consciousness and awareness of the individual pervades all of that person's experience, to such an extent that absolute notions of 'inside' and 'outside' are arbitrary. The universe is the same as the self, as the universe can only be experienced through the self and the self is submerged within the universe as an integrated part.
Aham Brahmasmi. Translation: aham = I; Brahmasmi = am Brahman. Meaning: I am Brahman.
It is an expression of "I am ness"....
We can discern at least five different states of "I am ness" in beings (jivas). The fifth one is the state of "I am I am"....This is a pure state of "I am only". This is the indescribable state of Brahman in its absolute aspect....
In Atman every experience is subjective. Atman does not interact with any object or use any external means to know or experience. It exists by itself and knows by itself. In contrast the experience of elemental self is objective....
All is Brahman. There is nothing else other than Brahman. He is the cause of all causes. He is also called Atman. In reality there is no distinction between the two. Atman is another name of Brahman or a mental construct we use to make sense of Brahman as the self of individual beings. There may be many beings in the creation, but there is only one Brahman in all of them. During creation Brahman projects himself out as everything and at the end of creation withdraws everything into himself. The phenomenal world is unreal. It an illusion or an apparition, which disappears when we overcome our ignorance and realize the true state of Brahman.
Aham Brahmasmi, by Jayaram V, Hindu Website,com
However, Advaita is strongly divergent from solipsism in that the former is a system of exploration of one's mind in order to finally understand the nature of the self and attain complete knowledge. The unity of existence is said to be directly experienced and understood at the end as a part of complete knowledge. On the other hand solipsism posits the non-existence of the external void right at the beginning, and says that no further inquiry is possible. [???]
Yogic practices are sometimes seen to align closely with the Sankhya philosophy, which is an Eastern dualistic system (somewhat distinct from Western dualism) postulating only the existence of mind, and of matter. However, one sometimes sees it explained that, while matter exists for us in the world of Maya (illusion), it is ultimately a product of mind, and is encompassed thereby.
The Buddha stated, "Within this fathom long body is the world, the origin of the world, the cessation of the world and the path leading to the cessation of the world." Whilst not rejecting the occurrence of external phenomena, the Buddha focused on the illusion created within the mind of the perceiver by the process of ascribing permanence to impermanent phenomena, satisfaction to unsatisfying experiences, and a sense of reality to things that were effectively insubstantial.
Mahayana Buddhism also challenged as illusion the idea that one can experience an 'objective' reality independent of individual perceiving minds.
According to the Sutra Prasangika view, external objects do exist, just not inherently: [LOL!] "Just as objects of mind do not exist [inherently], mind also does not exist [inherently]." In other words, even though a chair may physically exist, individuals can only ever experience it through the medium of each their own mind, with each their own literal point-of-view. Therefore, an independent purely 'objective' reality could never be experienced.
Some later representatives of one Yogacara subschool (Prajnakaragupta, Ratnakirti) were proponents of extreme illusionism and solipsism (as well as of solipsism of this moment). The best example of such extreme ideas was the treatise of Ratnakirti (11th century) "Refutation of the existence of other minds" (Santanantara dusana).
Note: It is important to note that all mentioned Yogacara trends are not purely philosophical but religious–philosophical. All Yogacara discourse takes place within the religious and doctrinal dimension of Buddhism. It is also determined by the fundamental Buddhist problem, that is living being and its liberation from the bondage of Samsara.
There are a number of critiques of, and responses to, solipsism.
The person dies, but the solipsist himself or herself is not dead. If somebody else dies, the supposed being who has supposedly "died" is only a phantom of the solipsist's imagination anyway, and the elimination of that phantom proves nothing. A critic might point out that many (self-proclaimed) solipsists have died in the history of the world, and the universe hasn't disappeared yet. However, the solipsist would respond that he or she has not died, and therefore his or her solipsism is not yet disproved.
For the solipsist, death can only ever be "the death of the other." He/ she never believed in the existence of those other solipsists in the first place. Note however that a sentence reflecting a supposed understanding of the actions of others, such as one that contains a statement like "he or she never believed..." or "those others" is already stepping outside of the solipsist's subjective viewpoint, and so misleading when used to illustrate a solipsistic view of death. For the solipsist, death is like the end of a dream. The dream itself, the setting, and any characters within that dream would not be thought to continue after waking. Ergo, if waking life is like a dream, why should the setting or characters perceived within it continue on after the death of the subject from that dream either?
Some might argue that to exist in complete unity with reality would mean to be unable to learn -- one would have to have awareness of all things. The solipsist would probably not appeal to such knowledge being contained in their subconscious, since the existence of a world of information outside consciousness is exactly what the solipsist disavows. Instead she might suggest that the world that exists is always restricted to her field of vision -- the way no world exists behind the walls of a stage's set, video game environment, or in a dream.
Applicability of the past
The fact that an individual may find a statement such as "I think, therefore I am" applicable to them, yet not originating in their mind indicates that others have had a comparable degree of insight into their own mental processes, and that these are similar enough to the subject's. The metaphysical solipsist would respond that, much like other people are products of his own mind, so, too, is "the past" and its attendant information. Thus, "I think, therefore I am" would indeed have originated in their mind.
Life is imperfect
Why would a solipsist create things such as pain and loss for himself or herself? More generally, it might be asked "If the world is completely in my head, how come I don't live the most fantastic life imaginable?" One response would be to simply plead ignorance and note that there may be some reason which was forgotten on purpose. Perhaps this is all out of a desire to avoid being bored, or perhaps even that the solipsist is in fact living the most perfect life he or she could imagine. Another response is that categories such as 'pain' are perceptions assumed with all of the other socio-cultural human values that the solipsist has created for himself — a package deal, so to speak. A third response is to say that, like a dream, the solipsist's subconscious mind creates a world which the solipsist's conscious mind might not have chosen but has no control over changing.
This issue is somewhat related to theodicy, the "problem of evil", except that the solipsist himself is the all-powerful God who has somehow allowed imperfection into his world. A solipsist may also counter that since he never made himself he never had a choice in the way his mind operates and appears to have only limited control over how his experiences evolve. He could also conclude that the world of his own mind's creation is the exact total of all his desires, conscious and otherwise and that each moment is always perfect in the sense that it would not be other than as his own mind in total had made.
The imperfection of life can also be explained through the belief that only through pain, both physical and emotional, can one move to a higher state of existence. Thus, it could be theorized that the imperfect present for a solipsist is the direct result of his subconscious compulsion to experience perfection.
A variant of this problem questions the existence of other people's skills the solipsists lacks. If the solipsist created a famous poet in his mind, why doesn't the solipsist have the capacity to imitate their skill? Similar to pain, there is some reason that the solipsist has denied himself this ability, but it may not be knowable or explainable.
The solipsist would say it is better to disregard the unreliable observations of alleged other people and rely upon the immediate certainty of one's own perceptions ...Solipsism agrees with Descartes in this aspect, and goes further: only things that can be known to exist for sure should be considered to exist.
-- Solipsism, by Wikipedia
The claim that the solipsist's mind is the only thing with certain existence for him (epistemological solipsism) does not inherently address the question of control over the content of that mind. Outside solipsism, a person may know that a phobia is all in the mind but be completely unable to prevent it ruining their life. (Conversely, it is not illogical for a powerful being—a god, for example—to have complete control over the universe, despite it being external to said powerful being.) Solipsism asserts that the mind of the agent is the only thing with assured existence; it need not assert any specific structure to that mind—any more or less than materialism—in and of itself, and requires a specific cosmology. However, any convincing philosophy needs to cohere with what is observed, and metaphysical solipsism needs to credit certain mental contents with the same stubborn indifference to human wishes that material objects display in other philosophies.
Solipsism undercuts morality
If solipsism is true, then practically all standards for moral behavior would seem to be meaningless. According to this argument there is no "creator", no deity, so that an external, "objective" basis for morality is gone. Other forms of morality that do not rely on the existence of an infallible deity, such as secular humanism, also become meaningless because there are no such things as other humans. Everything and everyone else is just a figment of imagination, so there's no particular reason not to make these figments disappear by, say, mass annihilation. The problem with this argument is that it falls prey to the Appeal to Consequences Fallacy; whether or not solipsism is true does not depend on its implications. This can possibly be countered by people who believe that (a non-solipsist) morality is an inherent part of the universe that can be proven to exist.
A solipsist may also understand that everything being a part of himself would also mean that harming anything would be harming himself with associated negative consequences such as pain (although the solipsist must be harming himself already, since "life is imperfect"). Or an exponent of a weak form of solipsism might say that harming others is imprudent because the solipsist can only be uncertain of their real existence rather than certain of their non-existence. Another expression of this point is in noting the strong feelings that a human can have for a non-existent character in a movie, or for a car or boat which is admitted to be completely non sentient. There is no logical or psychological reason to prevent a solipsist caring for observed people, even if the solipsist is completely convinced of their non-existence.
The solipsist needs a language
The practical solipsist needs a language to formulate his or her thoughts about solipsism. Language is an essential tool to communicate with other minds. Why does a solipsist universe need a language? Indeed, one might even say, solipsism is necessarily incoherent, a self-refuting idea, for to make an appeal to logical rules or empirical evidence the solipsist would implicitly have to affirm the very thing in which he or she purportedly refuses to believe: the 'reality' of intersubjectively valid criteria, and/or of a public, extra-mental world. A possible response would be that to keep from becoming bored, perhaps the solipsist imagines "other" minds, which would actually be only elements of his own mind. He or she has chosen to forget control of these minds for the time being, and the elaborate languages required for interaction with these more isolated segments of his mind are merely part of the creation of "reality." As for the rules of logic, they are probably merely an artifact of the peculiar psychology of the solipsist and only appear to exist in the "real" world. (However, to argue this way is to admit that solipsism needs to be buttressed with additional, ad-hoc hypotheses).
One famous argument along these lines is the private language argument of Wittgenstein. In brief, this states that since language is for communication, and communication requires two participants, the existence of language in the mind of the thinker means the existence of another mind to communicate with. There is a direct fallacy in this: either, language is for communication between two agents, in which case it is still to be proved that what is in the head of the agent is a language; in which case it is yet to be proved that language is for communication between two minds. To complicate the situation, the language in the mind of the agent may be for communication between the agent at this time, and the agent at a future time. However, this is no objection to the original argument, which explicitly mentions a kind of "diary" and therefore communication across time.
Similar to the above objections, the response that the solipsist, even being the only real thing, is not in control of the 'universe' could address this question.
Some philosophers, notably Bertrand Russell, hold the viewpoint that solipsism is entirely empty and without content. Like a 'faith' argument, it seems sterile, i.e., allows no further argument, nor can it be falsified. The world remains absolutely the same — so where could a solipsist go from there? Viewed in this way, solipsism seems only to have found a facile way to avoid the more difficult task of a critical analysis of what is 'real' and what isn't, and what 'reality' means. The solipsist might hold in response that further argument is meaningless. He might explain that, granting only the most basic laws of thought, he has identified the real limits of what can be truly known about 'reality': cogito ergo sum.
Solipsism amounts to realism
An objection, raised by David Deutsch (among others), is that since the solipsist has no control over the "universe" she is creating for herself, there must be some part of her mind, of which she is not conscious, that is doing the creating. If the solipsist makes her unconscious mind the object of scientific study (e.g. by conducting experiments), she will find that it behaves with the same complexity as the universe described by a realist. Thus what realism calls "the universe", solipsism calls "one's unconscious mind." Understood this way, the distinction between realism and solipsism collapses and amounts to different ways of describing the same thing: a massively complex process that causes all of the solipsist's experiences, but is not identical to the solipsist's conscious mind.
Presumably having made the case that the solipsist scientist is actually a realist scientist, Deutsch next argues in favor of the more common understanding of reality. He applies Occam's Razor, and suggests that it prefers the standard external 'reality' over something like a brain in a vat. This is because the standard 'reality' fits all the data available to the scientist, rendering superfluous the other more complicated possibilities.
If seeking to avoid rejecting the laws of thought, the solipsist may appeal to the problem of induction to reiterate that the realist's theory of reality could still, in the end, be an illusion in some way. She could also appeal to some types of idealism.
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