Like Descartes, Kant set for himself the formidable task of reviewing all knowledge in order to answer the questions: What can I know? What ought I do? What may I hope for?
The first of these questions, the nature and validity of human knowledge, he investigates in his book, The Critique of Pure Reason. Though he proceeds in an entirely different manner, he arrives at the same conclusion as Descartes: that man can never have direct and certain knowledge of anything outside his own mind.
We cannot, he says, know reality, things as they are in themselves, but only the appearances of things, because our faculties impose their own forms upon that which enters the mind. Neither sensation, judgment nor reasoning can give us a valid picture of the world outside ourselves. We can never reach the noumenon, the thing as it is in itself, but only the phenomenon, the thing as it appears to us.
Neither science nor philosophy can enable us to reach the substance, or essence of things. Nor can they tell us what the soul is, what matter is, or what God is.
Consequently, Kant teaches us another type of judgment called synthetic a priori, which leads to scientific knowledge. It enjoys the universality and necessity of analytic judgments without being tautological, and possesses the fecundity of synthetical a posteriori judgments, without being restricted to the particular beings existing in the empirical world.
For the formation of any synthetic a priori judgment it is necessary to have form and matter. The form is given by the intellect, independent of all experience, a priori, and signifies the function, manner, and law of knowing and acting, which the subject finds in itself prior to all experience.
The matter is the subjective sensations, which we receive from the external world. Thus, in The Critique of Pure Reason, Kant makes the essential elements of all knowledge (universality and necessity) dependent, not on the content of experience, but on a priori forms.
He shows that the truths that have always been considered the most important in the entire range of human knowledge have no foundation in metaphysical (or purely speculative) reasoning.
Therefore, what can I know? I can know the absolute knowledge of things, or causality. But this knowledge can only operate within the confines of the world – through what we are made to perceive with our senses. I cannot possibly know the noumenon, or the world-in-itself.
Like Descartes, Kant had attempted to tear down the whole structure of previously existing human knowledge, only to build it up again on what he considered a more solid foundation. For him, this foundation was the imperative command of conscience, which every man experiences in himself.
For Kant, the ultimate test of truth is not found in external reality, but is something in man’s own mind. This command of conscience, he declares, imposes itself with insistence and with certainty. Once the voice of conscience tells me that “I ought” to do something, I cannot escape from the certainty that I am obliged to do what my conscience commands. However, I can choose to obey or to disobey. In The Critique of Practical Reason, Kant makes the universality and necessity of the moral law dependent, not on the empirical act and the end we might intend in our actions, but on a categorical imperative, in the will itself.
For an act to be morally good, the will must be autonomous; it must be determined to act, not in view of the result of its action but only in view of its duty. Thus, duty for duty’s sake. This is Kantian morality. This means that among all the imperatives that can determine the will to action, it is necessary to distinguish the hypothetical from the categorical. Also, we cannot attain the suprasensible (noumenon) because our forms of knowledge (categories) are empty: their content is only phenomenal, conditioned matter.
Now, instead, the form of the will (categorical imperative) possesses the content independently within itself; it is not conditioned by any material element. It is the will itself that makes the human act morally good, and not vice versa. In fact, according to Kant, the empirical act will be good on condition that it be done for the sake of duty. Hence the will is beyond the phenomenal and mechanical world; it pertains to the world of noumena, of the unconditioned.
Thus, these truths rest on a solid moral basis, and are thus placed above all speculative contention and the clamour of metaphysical dispute. He has overthrown the imposing edifice which Cartesian dogmatism had built on the foundation “I think”; he now sets about the task of rebuilding the temple of truth on the foundation “I ought.” The moral law is supreme.
In point of certainty, it is superior to any deliverance of the purely speculative consciousness; I am more certain that “I ought” than I am that “I am glad”, “I am cold”, etc. In point of insistence, it is superior to any consideration of interest pleasure or happiness; I can forego what is for my interest, I can set other considerations above pleasure and happiness, but if my conscience tells me that “I ought” to do something, nothing can gainsay the voice of conscience, though, of course, I am free to obey or disobey.
This, then, is the one unshakable foundation of all moral, spiritual, and higher intellectual truth. The first peculiarity of the moral law is that it is universal and necessary. When conscience declares that it is wrong to tell a lie, the voice is not merely intended for here and now, not for “just this once”, but for all time and for all space; it is valid always and everywhere. Therefore, what must I do? My duty.
Now, what may I hope for? In order to understand this question, we have to consider Kant’s philosophy of religion, or critiqued, that is, the application of reason. In other words, the question: when I, as a finite being, have done what I should, then what may I hope for? – can be raised without assumption and honestly only when the human being as a finite, rational being knows both his/her limits and (therefore) freedom.
Thus, one has to accept a teleology merely as ‘the result prescribed by law’ rather than a motive of moral conduct, thereby the ‘summum bonum’ is postulated as an a priori synthesis of virtue and happiness. And for the sake of this synthesis is postulated a supreme being of holiness, grace, and justice – that is, God. Hence, morality leads unavoidably to religion and hope only starts with religion. Therefore, what may I hope for? God.
Thus, Kant’s three questions: 1) What can I know? 2) What must I do? And 3) What may I hope for? – become thematicized as metaphysics, ethics, and philosophy.