Hence the universal characteristic of the celestial bodies is that they move in a diurnal circle.
But we know that this one great fact which is true of them all has in reality nothing to do with them. The diurnal revolution which they visibly perform is the result of the condition of the observer. It is because the observer is on a rotating earth that a universal statement can be made about all the celestial bodies.
The universal statement which is valid about every one of the celestial bodies is that which does not concern them at all, and is but a statement of the condition of the observer.
Now there are universal statements of other kinds which we can make. We can say that all objects of experience are in space and subject to the laws of geometry.
Does this mean that space and all that it means is due to a condition of the observer?
If a universal law in one case means nothing affecting the objects themselves, but only a condition of observation, is this true in every case? There is shown us in astronomy a vera causa for the assertion of a universal. Is the same cause to be traced everywhere?
Such is a first approximation to the doctrine of Kant’s critique.
It is the apprehension of a relation into which, on the one side and the other, perfectly definite constituents enter—the human observer and the stars—and a transference of this relation to a region in which the constituents on either side are perfectly unknown.
If spatiality is due to a condition of the observer, the observer cannot be this bodily self of ours—the body, like the objects around it, are equally in space.
This conception Kant applied, not only to the intuitions of sense, but to the concepts of reason—wherever a universal statement is made there is afforded him an opportunity for the application of his principle. He constructed a system in which one hardly knows which the most to admire, the architectonic skill, or the reticence with regard to things in themselves, and the observer in himself.