The First Analogy concerns the category pair substance and accident. The concepts themselves are discussed under the lexicon for 'substance.' Substances and their accidents are thought under the schemata permanence and transitoriness, respectively. Thus a substance endures through time while the accidents of a substance appear and vanish over the course of time. One might think that the principle of substantiality would be that every appearance is a substance. However, Kant went further and tried to prove that all change is change of accidents in substances. This is a stronger claim, because it implies that substances cannot come into existence or be annihilated.
Such a view has been held by some philosophers, in one version or other. The atomists believed that atoms, by virtue of their indivisibility, are indestructible. Spinoza held that there is a single substance which exists necessarily. On the other hand, the Christian doctrine of the creation of the world ex nihilo conflicts with Kant's thesis. In Leibniz's doctrine, finite substances are all created together with the creation of the world, but once the world has been created, they persist throughout its history.
A Leibniz-style argument for the claim that substances cannot come into existence by natural means is instructive, because in some ways it parallels Kant's. If a substance were to be come to be from nothing, there would be no sufficient reason for its coming to be, since nothing cannot be a reason for something. If it is objected that there is a reason in some existing substances, the response is that the only power a finite substance has is over its own state. Clearly this argument is metaphysical, applying to things in themselves. Kant tried to field an argument whose scope is limited to appearances in space and time.
The argument is by reductio. Suppose a substance were to come into existence ex nihilo. Then there would be a time at which that substance does not exist. Furthermore, in order for our experience to be a unity, the coming-to-be of the substance must be assignable to some determinate time. Time itself cannot be perceived, and assignment to a determinate time is possible only by reference to something permanent, as a "substratum" of change. If there were no substratum among the objects of experience, we could only appeal to the succession of our perceptions. But this succession does not show whether a change occurs in an object or whether it is merely change in the subject. Thus, to assign the supposed coming-to-be to a determinate time, something in our experience must be permanent.
A contradiction would ensue if it were denied that there is anything permanent in experience, but there is no need to issue such a denial. So to get a contradiction, Kant must show that it is impossible to assign the supposed coming-to-be of a substance to a determinate time. He tries to do so by claiming that the genesis of the substance would not belong to the time in which other substances exist, but would create a new time-series. This would conflict with the unity of experience. So given that experience is unified, a supposed coming-to-be of a substance is impossible.
The obvious objection to this line of argument is that Kant's assertion, that a new substance would generate a new time, is gratuitious. That is, there is no reason to think it is true. Why can the genesis be marked at a given time by reference to existing substances?