T 1.4.5.5

If instead of answering these questions, any one shou'd evade the difficulty, by saying, that the definition of a substance is something which may exist by itself; and that this definition ought to satisfy us: Shou'd this be said, I shou'd observe, that this definition agrees to every thing, that can possibly be conceiv'd; and never will serve to distinguish substance from accident, or the soul from its perceptions. For thus I reason. Whatever is clearly conceiv'd may exist; and whatever is clearly conceiv'd, after any manner, may exist after the same manner. This is one principle, which has been already acknowledg'd. Again, every thing, which is different, is distinguishable, and every thing which is distinguishable, is separable by the imagination. This is another principle. My conclusion from both is, that since all our perceptions are different from each other, and from every thing else in the universe, they are also distinct and separable, and may be consider'd as separately existent, and may exist separately, and have no need of any thing else to support their existence. They are, therefore, substances, as far as this definition explains a substance.