P21: All the things which follow from the absolute nature of any of God/Nature’s attributes have always had to exist and be infinite, or are, through the same attribute, eternal and infinite.

Dem.: If you deny this, then conceive (if you can) that in some attribute of God/Nature there follows from its absolute nature something that is finite and has a determinate existence, or duration, e.g., God/Nature’s idea in thought. Now since thought is supposed to be an attribute of God/Nature, it is necessarily (by P11) infinite by its nature. But insofar as it has God/Nature’s idea, {thought} is supposed to be finite. But (by D2) {thought} cannot be conceived to be finite unless it is determined through thought itself. But {thought can} not {be determined} through thought itself, insofar as it constitutes God/Nature’s idea, for to that extent {thought} is supposed to be finite. Therefore, {thought must be determined} through thought insofar as it does not constitute God/Nature’s idea, which {thought} nevertheless (by P11) must necessarily exist. Therefore, there is thought which does not constitute God/Nature’s idea, and on that account God/Nature’s idea does not follow necessarily from the nature {of this thought} insofar as it is absolute thought (for {thought} is conceived both as constituting God/Nature’s idea and as not constituting it). {That God/Nature’s idea does not follow from thought, insofar as it is absolute thought} is contrary to the hypothesis. So if God/Nature’s idea in thought, or anything else in any attribute of God/Nature (for it does not matter what example is taken, since the demonstration is universal), follows from the necessity of the absolute nature of the attribute itself, it must necessarily be infinite. This was the first thing to be proven.

Next, what follows in this way from the necessity of the nature of any attribute cannot have a determinate {existence, or} duration. For if you deny this, then suppose there is, in some attribute of God/Nature, a thing which follows from the necessity of the nature of that attribute—e.g., God/Nature’s idea in thought—and suppose that at some time {this idea} did not exist or will not exist. But since thought is supposed to be an attribute of God/Nature, it must exist necessarily and be immutable (by P11 and P20 C2). So beyond the limits of the duration of God/Nature’s idea (for it is supposed that at some time {this idea} did not exist or will not exist) thought will have to exist without God/Nature’s idea. But this is contrary to the hypothesis, for it is supposed that God/Nature’s idea follows necessarily from the given thought. Therefore, God/Nature’s idea in thought, or anything else which follows necessarily from the absolute nature of some attribute of God/Nature, cannot have a determinate duration, but through the same attribute is eternal. This was the second thing {to be proven}. Note that the same is to be affirmed of any thing which, in some attribute of God/Nature, follows necessarily from God/Nature’s absolute nature.