Solipsism (/ˈsɒlɪpsɪzəm/ SOLL-ip-siz-əm; from Latin solus 'alone' and ipse 'self') is the philosophical idea that only one's mind is sure to exist. As an epistemological position, solipsism holds that knowledge of anything outside one's own mind is unsure; the external world and other minds cannot be known and might not exist outside the mind.
Philosophical realism—usually not treated as a position of its own but as a stance towards other subject matters—is the view that a certain kind of thing (ranging widely from abstract objects like numbers to moral statements to the physical world itself) has mind-independent existence, i.e. that it exists even in the absence of any mind perceiving it or that its existence is not just a mere appearance in the eye of the beholder. This includes a number of positions within epistemology and metaphysics which express that a given thing instead exists independently of knowledge, thought, or understanding. This can apply to items such as the physical world, the past and future, other minds, and the self, though may also apply less directly to things such as universals, mathematical truths, moral truths, and thought itself. However, realism may also include various positions which instead reject metaphysical treatments of reality altogether.
Realism can also be a view about the properties of reality in general, holding that reality exists independent of the mind, as opposed to non-realist views (like some forms of skepticism and solipsism) which question the certainty of anything beyond one's own mind. Philosophers who profess realism often claim that truth consists in a correspondence between cognitive representations and reality.
The problem of other minds is a philosophical problem traditionally stated as the following epistemological question: "Given that I can only observe the behavior of others, how can I know that others have minds?" The problem is that knowledge of other minds is always indirect. The problem of other minds does not negatively impact social interactions due to people having a "theory of mind" – the ability to spontaneously infer the mental states of others – supported by innate mirror neurons, a theory of mind mechanism, or a tacit theory. There has also been an increase in evidence that behavior results from cognition, which in turn requires a brain, and often involves consciousness.
It is a problem of the philosophical idea known as solipsism: the notion that for any person only one's own mind is known to exist. The problem of other minds maintains that no matter how sophisticated someone's behavior is, that does not reasonably guarantee that someone has the presence of thought occurring within them as when oneself engages in behavior. Phenomenology studies the subjective experience of human life resulting from consciousness. The specific subject within phenomenology studying other minds is intersubjectivity.
Subjectivism is the doctrine that "our own mental activity is the only unquestionable fact of our experience", instead of shared or communal, and that there is no external or objective truth.
While Thomas Hobbes was an early proponent of subjectivism, the success of this position is historically attributed to Descartes and his methodic doubt. He used it as an epistemological tool to prove the opposite (an objective world of facts independent of one's own knowledge, ergo the "Father of Modern Philosophy" inasmuch as his views underlie a scientific worldview). Subjectivism accords primacy to subjective experience as fundamental of all measure and law. In forms like solipsism, it may hold that the nature and existence of every object depends solely on someone's subjective awareness of it. One may consider the qualified empiricism of George Berkeley in this context, given his reliance on God as the prime mover of human perception.