Kant’s slogan “[t]houghts without content are empty, intuitions without concepts are blind” is the core idea of the Critique. By unpacking this ten word phrase we can outline all the key ideas necessary to understanding his whole critical project. The slogan indicates what components Kant thinks are necessary in order to have a cognition that leads to knowledge while setting limits on the scope of what we can claim to know. It also serves as a summary of Kant’s critique of both the Rationalist and Empiricist traditions and provides the foundation for his own philosophy of Transcendental Idealism.
Intuitions without concepts are blind
Kant argues in the Transcendental Aesthetic section of the Critique that rather than being objective features of the world outside of us, space and time are "pure forms of intuition" that we bring to experience as the framework for all possible perceptions. As a consequence, space and time are not to be considered as real in themselves but as rather supplied by the mind. This means that the objects that are perceived as existing in space and time are not wholly independent of our faculty of sensation (our "sensible intuition" to Kant); we do not merely receive bare sensations and copy them in the mind. The “manifold” of perception is disorderly without the order we bring to it; we must supply a priori concepts under which to organize these perceptions.
In the Transcendental Analytic Kant proceeds to demonstrate that the understanding provides this a priori structure in the form of the categories. The categories function as the concepts under which all our intuitions are organized. Toward the end of the Deduction Kant argues that the cognition of objects is not possible outside the framework of the categories. All our intuition is sensible and is empirical whenever an object is given. Empirical cognition is the same as experience, which means that a priori knowledge of objects is only the knowledge of possible objects of experience, even though this knowledge is not derived from experience. Experience and concepts can correspond only if one makes the other possible. Clearly experience does not make the categories possible, since they are a priori concepts. Therefore, the categories must make experience possible.
Thoughts without content are empty
The other half of Kant’s slogan serves to limit the pretensions of reason. Kant claims that the pure categories on their own cannot yield an object of experience, thus they are “empty” without intuition to provide them with content. He argues that since experience is limited to appearances, there is a dualism inherent in the use of reason when it attempts to think of objects as something other than objects of experience. In other words, reason produces a divide between objects of appearance (phenomena) and the transcendental objects assumed to be the cause of the appearances (noumena). Empirical knowledge is restricted to the former category; we cannot know anything about the latter, because our intuitions are only connected to the object as it appears to us in experience.
Much of the Transcendental Dialectic can be read as a demonstration of what goes wrong when we try to take concepts that aren’t connected to intuitions and extrapolate from them to form substantive knowledge. The paralogisms show that rational psychology errs by taking the representation of the “I” as an object given in intuition that can ground a developed doctrine making claims about the reality and properties of the soul. Rational cosmology entangles itself if a knot of conflicting arguments ("antinomies") due to its assumption that the unconditioned condition of a temporal or causal series is demonstrable (or demonstrably nonexistent) through reason, even though this unconditioned cannot be given in experience. Rational theology takes the idea of the highest being as discoverable by reason, resulting in the ontological, cosmological, and physico-theological proofs of the existence of God. Each of these arguments fails because the existence of such a being cannot be deduced from the concept alone.
The rejection of Empiricism and Rationalism
So the slogan exemplifies Kant’s theory shows the scope of our knowledge, in both a positive and negative sense. But the slogan can also be read as a summary of Kant’s critiques of both Empiricism and Rationalism that one could argue guide the project of the Critique.
The Empiricism of both Locke and Hume held that the mind receives ideas (or impressions) and then retains as a more or less passive receptacle. These ideas can be combined or abstracted from to create new ideas, but the basic parts of our knowledge are received by the mind “as is.” Kant’s theory held of course that even the basically passive sensibility through which intuitions are received plays a constitutive role in those intuitions by supplying the spatiotemporal dimensions that they all must have.
Similarly, Kant’s slogan provides a nice example of his rejection of the a priori extravagances of the Rationalists. Descartes and Leibniz, for example, had built significant systems on the basis of deductive arguments based solely on the content of their concepts, without any reference to experience. The proofs of the existence of God, the substantiality of the soul, etc. were all doctrines that under Kant’s system were unsupported; they are thoughts without content. Instead, there has to be an intuition (gained almost exclusively through sense) in order to ground any knowledge, to give it content. The ideas of reason might serve as “regulative” ideas to give our investigations direction, but they cannot serve as “constitutive” elements of our knowledge.
Finally, Kant's slogan provides the theoretical base on which his own preferred philosophical system, Transcendental Idealism, is based. Transcendental Idealism in a nutshell is the idea that because the objects of experience are not entirely independent of us, we never have knowledge of things in themselves; instead our knowledge is limited to things as they appear to us. Because of this, we have to approach knowledge from the perspective that the world we assume exists independently of our perception is ultimately unknowable. But for Kant, acknowledging this fact doesn't entail that we have to endorse Empirical Idealism (i.e., Berkeley's theory that appearances or ideas are all that really exist); instead, Kant thought that Transcendental Idealism was the only way to ground a philosophically coherent realism about the external world.