While, then, the general idea of a theory of knowledge as based upon psychological analysis is the groundwork of the Treatise, it is a particular consequence of this idea that furnishes to Hume the characteristic criterion applied by him to all philosophical questions.
The terms wh: ch he employs in describing the aim and scope of his work are not those which we should now employ, but the declaration, in the introduction to the Treatise, that the science of human nature must be treated according to the experimental method, is in fact equivalent to the statement of the principle implied in Locke's Essay, that the problems of psychology and of theory of knowledge are identical.
His general theory of knowledge deriving from Kant and Reid, and including among other things a contaminatio of their theories of perception, 3 in no way sustains or mitigates his narrow view of logic. He makes no effective use of his general formula that to think is to condition.
Locke, when Cartesianism had raised the problem of the contents of consciousness, and the spirit of Baconian positivism could not accept of anything that bore the ill-omened name of innate ideas, elaborated a theory of knowledge which is psychological in the sense that its problem is how the simple data with which the individual is in contact in sensation are worked up into a system.
His system shows the influence of Kant's destructive criticism of the claims of Pure Reason, recognition of the value of morally conditioned knowledge, and doctrine of the kingdom of ends; of Schleiermacher's historical treatment of Christianity, regulative use of the idea of religious fellowship, emphasis on the importance of religious feeling; and of Lotze's theory of knowledge and treatment of personality.
The determination too of the sense in which Kant's theory of knowledge involves an unresolved antithesis is for the logical purpose necessary so far only as it throws light upon his logic and his influence upon logical developments.
The truth is that the habit of thinking exclusively from the standpoint of the theory of knowledge tends to beget an undue subjectivity of temper.
Negatively, his philosophy is a polemic against the Stoic theory of knowledge in all its aspects.
The result of the foregoing, however, is to show that, as soon as epistemology draws its conclusion, it becomes metaphysics; the theory of knowledge passes into a theory of being.
If the theory of knowledge thus passes insensibly into metaphysics it becomes somewhat difficult to assign a distinct sphere to logic (q.v.).