Locke’s lesson was a lesson in critical caution, and in this sense, his philosophy was interpreted by the Enlightenment. «After so much unfortunate wandering – wrote Voltaire – tired, exhausted and shameful of having sought so many truths and found so many chimeras, I returned, like the prodigal son to his father, to Locke; and I threw myself into the arms of a modest man, who never pretends to know what he does not know, who does not possess, in truth, immense wealth, but whose funds are secure, and who enjoys without ostentation the most solid possessions.» Similar praise can be read in the Preliminary Discourse to the Encyclopedia, written by d’Alembert: it can be said that Locke – says d’Alembert – «created metaphysics, just as Newton created physics». And this metaphysics is «what it effectively must be, the experimental physics of the soul». A, therefore ‘reasonable’ metaphysics, no longer participating in the ‘ spirit of system ‘ of metaphysical constructionism. Illuministic knowledge is aimed at practical purposes (another Baconian motif), it is an eminently ‘useful’ knowledge. And philosophy is considered (and lived) as an essential factor of demystification, liberation, and progress.
The polemic against metaphysical constructions continues in I. Kant, and in him, it reaches its definitive form. Kant defines as dogmatic all metaphysics that does not presuppose a criticism of the ability to know. The wrong of these metaphysics has been to have ventured into the field of the supersensible, without taking into account the fact that no knowledge is possible without the intervention of sensitivity, which alone can attest to the presence of the known object. In the face of such constructions, the onset of skepticism is inevitable, which moreover limits itself to detecting the failures of reason, but without a previous criticism of it.
Kant provides this criticism, in which he establishes the limits of validity of the operations of the mind, as well as describing the structure of the mind itself. The result of the criticism is that it limits the scope of scientific knowledge and knowledge in general to the world of phenomena, but leaves open the traditional (and ineliminable for it is inherent to man) field of metaphysics to a different use of reason, the practical one. In this field, man encounters the problems of the freedom of will, the immortality of the soul, the existence of God, and resolves them positively according to the rationality of the moral principles that regulate (‘must’ regulate) actions. It is not a theoretical solution (a knowledge), impossible given the results of the criticism, but a practical solution, a practical certainty. Thus we have two worlds, the world of nature with its scientific laws and the world of freedom with its rational faith and its access to the supersensible.
The conciliation between these two worlds is possible through the use of judgment, which ponders on the purposes it encounters in nature and thinks them according to the principle of an intentional causality that would act in nature itself. Finalism, however, is a principle of exposition and understanding, not of explanation: the conciliation of the two worlds, sensible and supersensible, while legitimate and somehow necessary to man, remains hypothetical and problematic. A world in harmony with moral action is conceivable through a form of judgment different from that of scientific ones: such judgment does not grasp the essence of things but hypothesizes that it is sensible. This insistence on the ‘as if’ from Kant is due to his concern not to give value a status similar to that of fact and thus fall into a form of determinism and thus to compromise the autonomy of the reason and the freedom of man.