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We all conceive that a thing which we know scientifically cannot vary; when a thing that can vary is beyond the range of our observation, we do not know whether it exists or not. An object of Scientific Knowledge, therefore, exists of necessity. It is therefore eternal, for everything existing of absolute necessity is eternal; and what is eternal does not come into existence or perish. 3.  Again, it is held that all Scientific Knowledge can be communicated by teaching, and that what is scientifically known must be learnt. But all teaching starts from facts previously known, as we state in the Analytics,1 since it proceeds either by way of induction, or else by way of deduction. Now induction supplies a first principle or universal, deduction works from universals; therefore there are first principles from which deduction starts, which cannot be proved by deduction; therefore they are reached by induction. 3.  Scientific Knowledge, therefore, is the quality whereby we demonstrate,2 with the further qualifications included in our definition of it in the Analytics,3 namely, that a man knows a thing scientifically when he possesses a conviction arrived at in a certain way, and when the first principles on which that conviction rests are known to him with certainty—for unless he is more certain of his first principles than of the conclusion drawn from them he will only possess the knowledge in question accidentally.4 Let this stand as our definition of Scientific Knowledge.