These general facts are sometimes termed principles or laws, and are sometimes spoken of as the relationships of facts. A theory, or hypothesis, consists in a supposition of relationships which have not yet been ascertained. Thus Newton, after discovering the great general fact of gravitation, supposed that there might be a sort of ether connecting bodies together, and acting as the medium of their attraction. This supposition of a relationship, or general fact, not yet ascertained, is a theory or hypothesis. So when Stahl supposed the principle called the _vis medicatrix naturæ_ to be in the soul, and when Cullen supposed that it exists in the nerves and produces in fever a spasm of the extreme vessels, they both put forth a mere theory. Cullen speaks of Stahl’s theory as being fanciful. It is so. But Cullen’s is just as fanciful, if by this word it is meant that it is unsubstantiated by fact. Cullen’s supposition is more plausible, it is true, than Stahl’s; but it is no nearer being a proved fact. There is often much indefiniteness in the use of the word _theory_. Thus the doctrine, or law of gravitation, as discovered by Newton is sometimes spoken of as his _theory_ of gravitation. It was _once_ his theory; that is, when it was a mere supposition in his mind. But when, by a series of observations it came to be a proved fact, it was no longer a theory. So the laws of the circulation of the blood, as discovered by Harvey, are sometimes erroneously spoken of as his theory of the circulation. Some theories are said to be founded on facts, while others are deemed to be very fanciful. But theory can never be said, strictly speaking, to be founded on facts. It has relation to facts, it is true, but in the attempt to explain their nature, it goes _beyond_ them over into the domain of conjecture. Every one who puts forth a theory, is apt to think that all previous theories are false, while his is proved to be conformable to facts. Dr. Cullen, in announcing his theory, or doctrine (as he styles it) of fever, which is from beginning to end a series of unproved assertions, says, “I flatter myself that I have avoided hypothesis, and what have been called theories.” So Dr. Rush, after framing a theory even more palpably fanciful, says, that he conceives the doctrine of fever that he has aimed to establish rests upon facts only.