In what manner Diodorus stated and defended his opinion upon this point, we have no information. We know only that he placed affirmations respecting the future on the same footing as affirmations respecting the past: maintaining that our potential affirmation--_May or May not be_--respecting some future event, meant no more than it means respecting some past event, viz.: no inherent indeterminateness in the future sequence, but our ignorance of the determining conditions, and our inability to calculate their combined working.[66] In regard to scientific method generally, this problem is of the highest importance: for it is only so far as uniformity of sequence prevails, that facts become fit matter for scientific study.[67] Consistently with the doctrine of all-pervading uniformity of sequence, the definition of Hobbes gives the only complete account of the Impossible and Possible: _i.e._ an account such as would appear to an omniscient calculator, where _May or May not_ merge in _Will or Will not_. According as each person falls short of or approaches this ideal standard--according to his knowledge and mental resource, inductive and deductive--will be his appreciation of what may be or may not be--as of what may have been or may not have been during the past. But such appreciation, being relative to each individual mind, is liable to vary indefinitely, and does not admit of being embodied in one general definition. [Footnote 66: The same doctrine as that of the Megaric Diodorus is declared by Hobbes in clear and explicit language (First Grounds of Philosophy, ii. 10, 4-5):--"That is an impossible act, for the production of which there is no power plenary. For seeing plenary power is that in which all things concur which are requisite for the production of an act,** if the power shall never be plenary, there will always be wanting some of those things, without which the act cannot be produced. Wherefore that act shall never be produced: that is, that act is _impossible_. And every act, which is not impossible, is _possible_. Every act therefore which is possible, shall at some time or other be produced. For if it shall never be produced, then those things shall never concur which are requisite for the production of it; wherefore the act is _impossible_, by the definition; which is contrary to what was supposed. "A _necessary act_ is that, the production of which it is impossible to hinder: and therefore every act that shall be produced, shall necessarily be produced; for that it shall not be produced is impossible, because, as has already been demonstrated, every possible act shall at some time be produced. Nay, this proposition--_What shall be shall be_--is as necessary a proposition as this--_A man is a man_.