The need to find a unifying principle for all knowledge, an original synthesis meant as an ‘a priori’ representation of all a man knows and as such precedes the consciousness itself of multiplicity, leads Kant to elaborate the doctrine of '' I think ', which is one of the most debated and significant point of his whole philosophy.
The different representations of my intellect are unified in the horizon of what I thought, because they are accompanied by the awareness that I think about them. The ‘ I think’ is therefore the supreme principle of all synthesis, i.e. the horizon which the synthesis made by the categories connect in a unified manner, and as well the principle of every knowledge whereby the mind is conscious of the created unification. The principle makes it possible a real unitary knowledge of reality and at the same time it takes root in the awareness of the constitutive human finitude: it is worth noting that, in this sense, the ‘ I think’ is an organizing principle, a transcendental structure that "must accompany" the representations of the subject, and not the principle from which the whole reality depends, as it will be understood later by idealist thinkers .
Fichte, for example, in a letter of 1793, would say of Kant, "this unique thinker becomes to me increasingly marvelous: I think he has a genius that shows him the truth, but without revealing the fundamentals." However, on his part, Kant is much careful to point out how the ‘I think’ is the structure of thinking of each empirical subject, and then as it does not coincide nor - in the wake of Descartes - with an ‘individual I’ object of immediate self-consciousness, nor - as suggested by Spinoza and taken by idealists - with the ‘absolute I’ that is the foundation of all finite consciousness.
Specifically, the problem that Kant sought to resolve, which he addressed in the transcendental deduction of the Critique of Pure Reason, was as follows: why nature seems to follow necessary laws by conforming to those of our intellect? By what right do the latter can say to know scientifically the nature, "establishing" the laws in one way rather than another?
According to Kant, such a right is justified because the foundation of our knowledge is not in the nature but in the activity itself of the subject.