Our fundamental ideas of Nature were called by Locke sensible ideas. These ideas were derived from our sensible Experience, and it is only just to Locke to point out that, when examined in detail, his sensible ideas are seen to be not mere qualifications of sensation, but rather the elementary characters of Nature viewed as a dynamic process and discovered by our Activity. Yet the ambiguous term sensible ideas unfortunately led to their being regarded as ideas derived, not from our action in any form, but from pure sensation alone.
This extraordinary error was intensified in the speculation of Berkeley and Hume. Experience with them appeared to consist solely of a succession of sensations appearing to, impressing, or affecting a tabula rasa of consciousness.
Of course in such a state of affairs all Knowledge would be impossible. The scepticism which logically followed from such a doctrine was too universal to be capable even of the fiction that it was credible. Berkeley, it is true, endeavoured to save the situation by postulating the incessant and immediate intervention of the Deity as the sustainer of the sensible panorama. This purely arbitrary and fictitious expedient was entirely rejected by Hume, who with fearless honesty carried to its ultimate results the direct consequences of the doctrine and then complacently left human Knowledge to take care of itself.
A masterly protest against the position of Hume was made by his countryman Reid, who in his Inquiry into the Human Mind very clearly pointed out the fundamental difference between the sensible accompaniments or constituents of our Experience and the real and independently existent substratum by which that Experience is sustained and organised. His argument, though it attracted considerable attention, did not, however, affect as deeply as might have been expected the future of philosophic speculation, probably because he offered no new clue or key whereby to detect the origin and account for the presence in our Experience of those enduring and substantial elements or forms by which it is sustained, but on the contrary left their recognition to what he rather vaguely described as common sense.
Much more influential was the elaborate answer of Kant, which has profoundly affected the course of Metaphysics since its publication. Reverting in principle to the platonic method, Kant again sought the enduring elements, the fundamentals of Science, in the constitution of the cognitive faculty itself. But very differently from Plato he discovered these in the categories or essential forms of intellective action,—the category of causality and dependence and the so-called forms of the transcendental æsthetic—Time and Space. Under these categories the indefinite data of sensation were thought to be organised into a cognisable system.
A rapid advance of speculation along the lines signalised by Kant took place after his work was published, and for many years this movement was regarded by a large part of the speculative world as the most hopeful and progressive of philosophic efforts, and by its own votaries as placing them in a position of superiority to all other schools of thought. The thoroughness of their studies and introspective methods to some extent justified, or at least excused the arrogance of their pretensions.