The theory of knowledge, also known as epistemology, is a branch of philosophy that investigates the nature, scope, and limits of knowledge.

René Descartes is widely regarded as the father of modern epistemology due to his influential work, 'Meditations on First Philosophy', in which he questioned the certainty of all knowledge and proposed the method of doubt as a means of reaching true knowledge.

Descartes' method of doubt involves questioning all beliefs, including those based on sensory experience, until one reaches a foundation of knowledge that is absolutely certain and indubitable.

Descartes' method of doubt led him to the indubitable truth expressed in the Latin phrase 'Cogito ergo sum', which means 'I think, therefore I am'.

John Locke argued that all knowledge is derived from sensory experience, which he called 'sensations' and 'reflections'.

Apperception is the process by which we organize and interpret sensory experience according to innate categories of thought, such as space, time, and causality.

David Hume argued that all knowledge is subjective and relative to the individual, and that there is no objective foundation for knowledge.

The problem of skepticism arises from the fact that our sensory experience is often unreliable and deceptive, leading to the question of whether we can ever truly know anything.

Immanuel Kant argued that knowledge is a combination of sensory experience and innate ideas, which he called 'categories of understanding'.

Kant made a distinction between things as they appear to us (the phenomenal realm) and things as they are in themselves (the noumenal realm).

Michel Foucault argued that knowledge is a social construction and that truth is relative to the community in which it is held.

Solipsism is the theory that holds that all knowledge is subjective and that there is no objective reality.

William James argued that the only way to truly know something is through direct experience.

The foundationalist theory of knowledge holds that knowledge is justified true belief.

Charles Sanders Peirce argued that knowledge is a process of ongoing inquiry and that there is no such thing as absolute certainty.