Reasoning is an argument in which certain things being laid down, something other than these necessarily comes about through them. It is a 'demonstration', when the premises from which the reasoning starts are true and primary, or are such that our knowledge of them has originally come through premises which are primary and true. Reasoning on the other hand is dialectical, if it reasons from opinions that are generally accepted. Things are 'true' and 'primary' which are believed on the strength not of anything else but of themselves: for in regard to the first principle of science it is improper to ask any further for the why and wherefore of them; each of the first principle should command belief in and by itself. On the other hand, those opinions are "generally accepted" which are accepted by everyone or by the majority or by the philosophers, that is, by all or by the majority or by the most notable and illustrious of them. Again reasoning is contentious if it starts from opinions that seem to be generally accepted but are not really such or again if it merely seems to reason from opinions that are or seem to be generally accepted. For not every opinion that seems to be generally accepted actually is generally accepted. For in none of the opinions which we call generally accepted is the illusion entirely on the surface, as happens in the case of the principles of contentious arguments; for the nature of the fallacy in these is obvious immediately, and as a rule even to persons with little power of comprehension. So then, of the contentious reasonings mentioned, the former really deserves to be called "reasoning" as well, but the other should be called "contentious reasoning", but not reasoning since it appears to reason, but does not really do so. Further, besides all the reasonings we have mentioned there are the misreasonings that start from the premises peculiar to the special sciences, as happens (for example) in the case of geometry and her sister sciences. For this form of reasoning appears to differ from the reasonings mentioned above; the man who draws a false figure reasons from things that are neither true and primary, nor yet generally accepted. For he does not fall within the definition; he does not assume opinions that are received either by everyone, or by the majority, or by the philosophers that is to say, by all, or by most, or by the most illustrious of them. But he conducts his reasoning upon assumptions, which though appropriate to the science in question, are not true; for he effects his misreasoning either by describing the semi-circles wrongly, or by drawing certain lines in a way in which they could not be drawn.