As far as the details go, I will try to abbreviate as much as possible and still convey the gist of Kant’s thinking with the CPR and then with the CPrR.

Schopenhauer has given us the idea (spelled with lower case deliberately, as will be understood later) the idea of the brainarium. As we all know light comes from the sun, strikes the tree, some of it is absorbed and some is reflect into my eye and an impression ensues which is reversed relative to the actual tree out in space (to be exposed later) and is converted into electrical impulses and sent along the optic nerve into the brain and there a projection arises (where the reversal is corrected by the brain), and in that projection we see shapes popping up to us, e.g., a tree or the face in the cloud, and these shapes, colored and toned as they may be, Kant calls appearances (Erscheinungen). I want to call this our brainarium, like a giant, 3-D cyclorama (see especially in the Cyclorama of Atlanta’s Grant Park). The rainbow and the rain that we spy in the distance are both a projection of our brains and as such do not have any existence at all when we are not looking at them, e.g., when we blink. Indeed even the space and time in which we view these appearances are nothing more than the way we look at the appearances in the brainarium, for certainly here and there and now and then are never in the appearances at all, but only in the way that we look at them.

First Hume and then Kant asked this question: since all that we can ever know or even imagine is prompted and given always only and entirely through the brainarium, and since there is nothing abiding or enduring or necessary about the appearances, popping up as they do, how did we ever come to the idea that these appearances were not all there is, but that there is something else, something that these appearances merely represent and stand for (Vorstellung). Obviously (and both Kant and Hume realized this) this something else is something that we ourselves have added to the brainarium. This understanding contradicted the very system of human knowledge that Hume had erected, and so he became an academic skeptic. Hume had to give up. Kant refused to give up.

Kant looked at geometry and thought about a triangle and realized that there is nothing in the concept of a triangle (namely three straight line segments with every endpoint common to two of these lines) that tells us that every side is shorter than the other two combined. Hume thought mathematics was analytical, and and we see here that he is wrong, for we see that the knowledge of this property of the sides cannot be gleaned by any analysis of the of the concept of the triangle. But Hume and Kant both knew this knowledge could not be empirically obtained by inspection, i.e., it could not be that we learn about triangles the way we learn about tables or toothbrushes, for that only tells us what we have experience thus far, and not that this property of the length of the lines must be this way. And so it is true that every triangle we have spotted drawn out on paper or spied in crossing telephone wires on poles is so constituted that every two of the sides are always greater than the third. Thus the only way we can know this, says Kant, is if we ourselves provide the object, i.e., construct one in space, for it is then that we first realized that two sides always are greater than the third, for without that we couldn’t have a triangle. And so this knowledge was what Kant called both synthetic, i.e., based on an exposure to an actual object, and a priori, independently of any empirical experience with the object. And what we do is to exercise our marvelous capacity for envisaging or looking and viewing something before our eyes, e.g., a face in the cloud. So we can trace out in mid air the outline of a triangle, and we are able to “see” that triangle, by our imagination, not just picturing it in our head, but placing a triangle in space before us and actually spying it in space before us and where we are able to point it out to others also to see. Even though all that is there is a so-called pure envisagement/view (Anschauung).

So we have the capacity to provide objects to the brainarium and this capacity of ours is valid for geometry, for it is only in this way that our recognition of these objects can be represented and sighted in pure space and time (as our triangle just above). We recognize them because we have put them there and have actually then seen them there, be it the imaginary triangle or the face in the cloud or the tree. Kant maintains that had Hume realized this fact of our knowledge of mathematics, he would have realized that it was synthetic and a priori, and he then would have figured out the CPR before Kant and would have expressed much better.

Now, Kant says to Hume, we know that we cannot fix the size of your famous table by looking it, for all we get are changes, growing smaller and larger depending of the distance from us. And yet we both know that the table does not change and that we are looking at merely the appearances of this unchanging table. Since we can’t get this knowledge of this uniformity of existence from experience, and yet we know it anyway, it follows that we have contributed the object (the table) ourselves, just as we did with the triangle. And so what we have is this: we provide the “object of experience” which then the appearance of the table in the brainarium represents to us. By means of this object of experience, this ‘real table’, we come to recognize the appearances as such, i.e., we come to realize that all we see and sense is limited to the brainarium. And so our presumption in providing this object of experience, this notion of a flat surface elevated by legs, leads us to recognize the table and that it is abiding and that it is only our view of it that changes.

The details of this are involved. We have a mental capacity called the understanding. And this is a capacity for making connections, of putting two and two together into a four. There are various connections that we can make, like cause and effect.

So we have this connective understanding as an a priori capacity (preceding all experience) of making connections. And by our very nature as understanding beings we make the premise that all appearances are connected in one way or another, remotely or directly. This is the single most comprehensive and fundamental and private assumption we ever make with regard to empirical knowledge. Base on this assumption we anticipate connections and and so are alert to the hint of that such that when anything appears to us we are open to the possibility of connection. As a consequence of this we engage in what is called “paying attention”. We do this in anticipation of a connection. It is by virtue of our categorical understanding of connective laws of necessity that we are on the look out for the possibility of a connection and are able to pay attention to what’s going on.

We assume the constant size of Hume’s table as one of the connective devices of our understanding which is called uniformity in the quantity of matter, and then we look for a connection between the different appearances and find them in the perspective of ourselves as an on-looker, and so where the varying appearances (of the table) are all unified as different appearances or looks of one and the same thing. This is a synthetic endeavor and the object of constancy is provided by the capacity for understanding and connecting. And the recognition arises in seeing the diverse appearances as simply our own representations of these real things as appearances in the brainarium.

So briefly then regarding the table, the presumption of universal connection of all appearances in the brainarium leads to noticing certain coincidences, and then to experimentation and finally to the conception of the appearances in the object such that the appearances are all bound together as various representations of the one and unchanging object.


So Kant has shown the propriety in our presumption in the provision of objects to the world of the brainarium by showing its necessity in our realization and recognition of the brainarium. Now Kant wants to see what happens when we seek to do this with regard to objects apart from any possible brainarium. And this is the primary purpose of his critique of pure reason. Pure reason itself ventures out beyond the world of a possible brainarium into a realm of pure Ideas (and different now in meaning from ‘idea’ with no capital) and draws what it sees as very plain and necessary conclusions.

This analysis becomes much more involved. In the first place apart from the brainarium, where our concepts and dreams can be tested, and in the realm of Ideas (where the brainarium can provide no touchstone of our presumptions) we just have to make sure we don’t contradict ourselves in our thinking. Here we find it is natural for pure reason to lead us to the Ideas of the soul, free will and God (among others). Kant shows in detail how it is that we are dealing with a confusion which is similar to the mirage we see of water on the distant road which always vanishes when approached. This confusion (actually an illusion) arises by leaving the realm of the appearances of the brainarium and sailing out in thought to consider Ideas.

Very briefly it goes like this. We know there is something apart from the brainarium, such that our appearances are just that, i.e., representations and not things on their own. This is the assumption we make for the appearances, that they are altogether representations of a single nature, and thus that they are all connected. So we conceive of a something, the “thing on its own” apart from the brainarium, and assume that it is subject to the laws of nature and so treat it as the object of experience which can be represented by appearances. But that does not affect the thing on its own, the fact that we are limited to the brainarium and thus to the connective laws of nature. And so we properly think of the thing on its own as entirely independent of all human looking, or looking of any kind (of sense). But then when we want to speak about the thing on its own in the sciences of the brainarium we are actually speaking of the object of experience, i.e., anything to the extent it appears in the space and time of a possible human brainarium in accordance with laws of a single nature.

This is important and needs to be grasped. We conceive of the thing on its own in order to deal with the appearances, but then in dealing with the appearances we think not of the thing on its own and entirely independent from all human looking, but of the object of experience. The confusion and illusion arises by speaking of the thing on its own as being the same as the object of experience, but is true with regard to science but is false with regard to things independently of the brainarium.

With regard to God, for example, we conceive of a being which is absolutely perfect, and we (and especially Descartes) assert that an object or being that exists is obviously more perfect than an object which is merely thought, and so we conclude, in pure reason, that such an absolutely prefect being must exist, for else it would contradict its own concept (where existence is a predicate like power and knowledge). Indeed we are also led to the necessary being by pure reason looking for the ultimate cause of the brainarium world. There must be such a being and what better being could there be besides this absolutely perfect being. So pure reason is satisfied in its conclusion of the necessary existence of God.

The main problem with this reasoning is the misuse of the term “is” as the expression of existence. The little word “is” is a connective device as in “the tree is tall”; it is not a predicate, as though one might say, “the tree is is”. So we can dream up perfect beings all we want to, there is still a question as to whether such a being exists such that we could rightly say, “that thing is”, i.e., it exists. And if we dream up a being with all predicates except one, and then we say that being exists, then we must mean that it exists with just those predicates and without the missing predicate, for otherwise we would be speaking of two different things, as though “a tree with leaves” and “the tree without leaves” were two different trees.

The upshot then of the CPR is that we are able to think the soul and free will and God consistently with the sciences of the brainarium, but we are not able to recognize them as existing objects, and the concepts we have of them, their Ideas, may be empty of any content, empty of any actual something corresponding to them. In a word, Kant finishes the CPR as an agnostic with regard to these three great, rational Ideas. We cannot say one way or the other that the soul, free will and God are anything more than grand Ideas.

Having finished with Pure Reason, Kant turns his attention to a critique of Practical Reason where he wants to discover here also the capacity and authority of reason in general to guide our actions. And so here we move from knowledge to actions and are concerned now with the will, the capacity of directing our actions in accordance with principles of reason. Is reason sufficient for directing our actions through this capacity of acting according to principles which is called will?

First Kant wants to establish the fact that pure reason itself is able to determine the will on its own and independently of any empirical object of desire. To do this he first uncovers that it would be impossible to have a universal law (which is the way that pure reason speaks and must speak as a universal voice for all rational being) in pursuit of happiness, for happiness is too variable and individual (“one man’s meat is another man’s poison”). The only possibility would be to eliminate all consideration of happiness and to so determine the action that the principle guiding the will to that action is expressed in such a way that it could serve as a universal law. That requirement is the moral law, a categorical (unconditioned) imperative that the principles of acting must always be subject to a universalization. And it is also made clear in this way that the only being who were capable of such a code of conduct would be a free being, one who could function independently and even in spite of all inclinations.

We all recognize this freedom, according to Kant, because we see ourselves as bound to this moral law of pure reason. Scientists recognize this freedom arising from this subjugation of the will to the moral law (even if people don’t always comply), and for that reason some have even wanted to include a study of freedom in the sciences, even though there is not the least justification for any such freedom to be found anywhere in the brainarium world (and which is of no use to science). And we are secure in this our recognition of our freedom because while we left the CPR as agnostic and simply unable to say one way or the other, now we do in fact recognize our freedom.

Having recognized the fact of our freedom we are not obligated to explain the possibility of freedom (which is impossible), for the fact proves the possibility. The question now has to do with the capacity of reason in general to guide us in all our actions including both the moral and the prudent (the principles guiding us with regard to our personal happiness). Since the prudent drive for personal happiness is often at odds with the categorical demands for morality, these two drives, happiness and morality, must be unified. This unification is only possible in the Idea of the Highest Good where we achieve to moral perfection (the demand of the moral law) and a happiness (demanded by our human nature) proportionate to, and a function of, that moral state. This Highest Good is not possible as a practical purpose of human rationality except under the conditions of an immortal soul and God. And so we lay claim to the existence of God and the soul, which was permitted but not given by the CPR.