Since the object of pure scientific knowledge cannot be other than it is, the truth obtained by demonstrative knowledge will be necessary. And since demonstrative knowledge is only present when we have a demonstration, it follows that demonstration is an inference from necessary premises. So we must consider what are the premises of demonstration—i.e., what is their character: and as a preliminary, let us define what we mean by an attribute ‘true in every instance of its subject …’ I call ‘true in every instance’ what is truly predicable of all instances—not of one to the exclusion of others—and at all times, not at this or that time only; e.g., if animal is truly predicable of every instance of man, then if it be true to say ‘this is a man,’ ‘this is an animal’ is also true, and if the one be true now the other is true now.
Where demonstration is possible, one who can give no account which includes the cause has no scientific knowledge.
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