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A Brief Note on Kant and How Synthetic Propositions a priori are Possible

Updated on October 30, 2009


In the Prolegomena to Any Future Metaphysics, Kant’s most important question is, “[h]ow are synthetic propositions a priori possible?” (4: 276). Kant proposes that we are aware of at least two sources of synthetic propositions a priori: propositions of pure mathematics and propositions of pure science. Beginning with pure mathematics, Kant asks a second, somewhat simpler, question, “[h]ow is it possible to intuit anything a priori? (Section 8, 4: 282). For Kant, intuition is sensible intuition, i.e., the image that is presented before one’s mind, not yet analyzed by the understanding. Intuition can also be thought of as a way of seeing. So here, Kant is essentially asking, ‘how can we know anything about an object prior to having any experience of it?’ Furthermore, how can we even know what object we are referring to if we have not yet met with this object in our experience? For example, how can I know that grass is green if I have never seen, been shown, or told about grass? I have to see, be shown, or be told about grass before I can know any of its properties. I cannot intuitively know that grass is green. Furthermore, I need to be acquainted with (i.e., have some form of experience of) grass before I can even know what it is we are talking about when we say the word “grass.” But, if I have to have experienced grass before I can even know what we are referring to when we say, “grass,” then I don’t know anything about grass a priori.


In response to this question, Kant proposes that Space and Time are features of our mind that we necessarily (but passively) rely upon to interpret everything. That is, our minds are structured in such a way that we process information by (unconsciously)  applying the forms Space and Time to everything; it is only through the “lense” of Space and Time that things are known to us. Space and Time are what Kant refers to as “forms of sensibility” (see §10, 4: 283). We know the form (i.e. the way in which things will be presented to us) of things a priori because all things known to us are known through the forms of sensibility (i.e., Space and Time). Thus we can know the form of anything a priori because everything we ever know we know through these forms of sensibility. For Kant, “forms of sensibility” refers to Space and Time, which are innate intuitions that determine the way the world appears to our sensibilities.


For example, regarding some new object X that I have never seen or heard of before, I can intuit a priori about this object the form in which it will be presented to me. I can intuit the form in which it will be presented to me because everything that is ever presented to me is presented in the same form (i.e., the form of the sensibilities, i.e., Space and Time). It is only through the forms of Space and Time that I am able to perceive anything at all. Hence I can intuit a priori many things about some future object of sense perception that I am, at present, unfamiliar with. For instance, I can intuit a priori that two Xs cannot occupy the same space at the same time; and I can intuit a priori that two Xs will exist in some spatial relation to one another. These are several types of things I can intuit a priori about objects I have not yet met with in sense experience. Thus, it is possible to intuit something a priori.


I believe Kant has been effective in his attempt to demonstrate that it is possible to intuit something a priori, and, furthermore, he has been successful in showing how this is possible. We only need to analyze our own intuitions to recognize that every intuition we have, we intuit through the forms of Space and Time. We cannot comprehend intuiting something without the forms of Space and Time. Thus, in my opinion Kant has correctly identified how it is possible to intuit something a priori.




Immanuel Kant, Prolegomena to Any Future Metaphysics ed., Gunter Zoller, (Oxford: Oxford University Press, 2004).



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