Aristotle (Seconds Analytics), Descartes (Rules for the Direction of Mind), Locke (Essay on the Human Understanding), Hume (Treatise on the Human Understanding), Kant (Criticism of Pure Reason), and Russell (Problems of philosophy, 1912, Theory of knowledge, 1913, Our knowledge of the outside world, 1914), have a theory of knowledge at two levels: a knowledge is either (a) a basic knowledge, or (b) a inferred knowledge of a basic knowledge. The basic knowledge is the first principles, those that are not derived from anything else. For Aristotle, these are very general principles that give the essence of a thing; for Descartes, a small number of truths grasped in a clear, distinct and unmistakable way; for Locke, the sensations; for Hume, sensible impressions; for Kant, the intuitions of the senses (or sensations) and the principles of the understanding that organize them; for Russell, the data of the senses and the principles of logic. The derived knowledge is science and our ordinary knowledge of the world. These theories are said to be foundational: a subset of our knowledge serves as a foundation for all our other knowledge.
These two-level theories seem to suggest that there is no single definition of knowledge, since a knowledge is either a first knowledge or a derived knowledge. But in fact, these theories are consistent with the traditional definition. We can indeed rephrase them as follows: knowledge is a true and justified belief, but there are two ways of being justified: (a) for basic beliefs, they are self-justified, (b) for derived beliefs they are justified because they are inferred from other beliefs that are justified.
This reformulation makes it possible to see how the definition presented as “traditional” in the preceding sections is indeed that adopted, often implicitly, by the majority of the great philosophers of the knowledge from Plato to Russell.
Definition as adequacy to the object
Other definitions of knowledge (in the philosophy of ancient perception, in Thomas Aquinas – “veritas est adequatio intellectus et rei” – in Hegel, in phenomenology) are based on the idea of adequacy of the knowing subject to the object.
Many philosophers have reserved the name of knowledge for exceptional epistemic states. For example, Plato calls “knowledge” (or “science”, episteme) the intuitive grasping of Shapes or Ideas of Things. Similarly, for Aristotle, there is “knowledge” and “science” (episteme) only of the general. If these restrictive definitions can be used to characterize science or to designate an exceptional cognitive state targeted by the philosopher, they amount to a strong distinction between the substantive “knowledge” of everyday uses of the verbs “to see” or “to know”: for example, to see where and when you are born, to see that it rained three times last week, to see that there is a table and two chairs in front of you, to see my neighbor Robert, etc.