PROP. XVI.————
Its downfall.render all cognition impossible. Unless the mind could know something without knowing anything more—in other words, unless it could know substance (for known substance, according to the definition, is whatever can be known without anything more being known), no knowledge, as has been stated in the demonstration, could arise; because, in that case, the mind, before it could know anything, would be eternally under the necessity of knowing something more; and this process never coming to an end, knowledge could never come to a beginning. But knowledge does come to a beginning; it takes place. Therefore the mind can know something without knowing anything besides; or, more shortly, it is cognisant of substance; and the counter-proposition which denies this truth can no more keep its ground against these considerations, than a soap-bubble can withstand a thunderbolt.
Defence of definition of known substance.7. A moderate degree of reflection may convince any one that the definition of known substance here presented, is the only true and tenable and intelligible definition of it which can be formed. No other conception of known or knowable substance can be formed than that it is that which can subsist in thought without anything else subsisting in thought along with it. Whatever can thus stand or subsist is certainly a known substance—a conceived subsistence; whether it be an existing sub-