Here two important points must be emphasized. The first, which has been stated before but should be repeated here, is the dependence of rational knowledge upon perceptual knowledge. Anyone who thinks that rational knowledge need not be derived from perceptual knowledge is an idealist. In the history of philosophy [p. 228] there is the “rationalist” school that admits the reality only of reason and not of experience, believing that reason alone is reliable while perceptual experience is not; this school errs by turning things upside down. The rational is reliable precisely because it has its source in sense perceptions, otherwise it would be like water without a source, a tree without roots, subjective, self-engendered, and unreliable. As to the sequence in the process of cognition, perceptual experience comes first; we stress the significance of social practice in the process of cognition precisely because social practice alone can give rise to human knowledge and it alone can start man on the acquisition of perceptual experience from the objective world. For a person who shuts his eyes, stops his ears and totally cuts himself off from the objective world there can be no such thing as knowledge. Knowledge begins[3-331] with experience – this is the materialism[3-332] of knowledge.
The second point is that knowledge needs to be deepened,[3-333] needs to be developed to the rational stage – this is the dialectics of the theory of knowledge. To think that knowledge can stop at the lower perceptual stage and that perceptual knowledge alone is reliable while rational knowledge is not would be to repeat the historical theory[3-334] of “empiricism”. This theory errs in failing to understand that, although the data of perception reflect certain realities in the objective world (I am not speaking here of idealist empiricism which confines experience to[3-335] introspection), they are merely one-sided and superficial, reflecting things incompletely and not reflecting their essence. Fully to reflect a thing in its totality, to reflect its essence, to reflect its inherent laws, it is necessary through the exercise of thought to reconstruct the rich data of sense perception, discarding the dross and selecting the essential, eliminating the false and retaining the true, proceeding from the one to the other and from the outside to the inside, in order to form a system of concepts and theories – it is necessary to make a change[3-336] from perceptual to rational knowledge. Such reconstructed knowledge is not more empty or more unreliable; on the contrary, whatever has been scientifically reconstructed in the process of cognition, on the basis of practice, reflects objective reality, as Lenin said, more deeply, more truly, more fully. As against this, vulgar “practical men” respect experience but despise theory, and therefore cannot have a comprehensive view of an entire objective process, lack clear direction and long-range perspective, and are complacent over occasional successes and glimpses of the truth. If such persons [p. 229] direct a revolution, they will lead it up a blind alley.
Rational knowledge depends upon perceptual knowledge and perceptual knowledge remains to be developed into rational knowledge – this is the dialectical-materialist theory of knowledge. In philosophy, neither rationalism nor empiricism[3-337] understands the historical or the dialectical nature of knowledge, and although each of these schools contains one aspect of the truth (here I am referring to materialist, not to idealist, rationalism and empiricism), both are wrong on the theory of knowledge as a whole. The dialectical-materialist movement of knowledge from the perceptual to the rational holds true for a minor process of cognition (for instance,[3-338] a single thing or task) as well as for a major process of cognition (for instance,[3-339] a whole society or a revolution).