Energy [is of two kinds: 1. Energy of motion; 2. Energy of position]. - William Kingdon Clifford

We have no right to believe a thing true because everybody says so unless there are good grounds for believing that some one person at least has the means of knowing what is true, and is speaking the truth so far as he knows it. However many nations and generations of men are brought into the witness-box they cannot testify to anything which they do not know. Every man who has accepted the statement from somebody else, without himself testing and verifying it, is out of court; his word is worth nothing at all. And when we get back at last to the true birth and beginning of the statement, two serious questions must be disposed of in regard to him who first made it: was he mistaken in thinking that he knew about this matter, or was he lying?

No mathematician can give any meaning to language about matter, force, inertia, used in text-books of mechanics.

In the cases of heat and electricity the form of the persisting energy is pretty well ascertained. But there are cases in which we do not know if it is energy of motion or energy position, such as that of . In the burning of coal there is a falling together of carbon and oxygen [and heat is produced] but we do not know in which of the two forms, if either, the energy which comes out as heat existed in the chemical process. For such a case the is only a probable statement (though of great probability) to the effect that in all cases where a physical quality is convertible with energy, that quality is itself either energy of motion or energy position.