So, and this is the important part, what was moral was identified with the conventional. What this meant is that the natural man, hidden inside the conventional man, was identified with the non-moral or pre-moral.
The next part is worth quoting in full:
"The natural man has no moral standards of his own. He is therefore free from all constraints upon him by others. All men are by nature either wolves or sheep; they prey or are preyed upon.
The natural man, conceived thus by the sophist, has a long history in European ethics in front of him. The details of his psychology will vary from writer to writer, but he is almost always - though not always - going to be aggressive and lustful. Morality is then explicable as a necessary compromise between the desire of natural men to aggress upon others and the fear of natural men that others will aggress upon them with fatal consequences. Mutual self-interest leads men to combine in setting up constraining rules to forbid aggression and lust...
A good deal of variation is possible in the way that this intellectual fairy tale is told, but its central themes, like those of all good fairy tales, are remarkably constant. And above all, at the heart of the account there remains the idea that social life is perhaps chronologically and certainly logically secondary to a form of unconstrained nonsocial human life..."
The problem with the view of the natural man that seems to have originated with the sophists is that it reduces the nature of men to a few basic, destructive instincts; that it sees the natural man as an atomised agent seeking his own selfish purposes; and that it severs the connection between the natural man and the collective institutions of society, with these institutions only existing as part of a social contract to constrain the destructive aspects of natural man.
It possibly also led to equally unhelpful counter-positions, in which the natural man living outside of convention was thought to be noble and only corrupted by conventional society, or in which the social institutions were thought to be a contract for the purposes of a few against the many and therefore oppressive, rather than being necessary constraints upon the natural man.
What is missing is a more nuanced few of human nature, one which sees men as having a moral nature, albeit a flawed one, so that men have it within their nature both to embody noble qualities as well as to pursue an aggressive self-interest. Nor does the natural man exist prior to human society - he has always been part of it. Institutions like the family or the tribe were not somehow contracted for but reflect the social nature and the social needs of the natural man. The family does constrain aspects of human nature, but it fulfils others at the same time.
You cannot sum up the nature of man in a line. You could write a whole library of books describing the biological, the intellectual, the moral, the spiritual, the social, the emotional, and the psychological impulses that run through men. Out of all of this, an individual and a culture attempt to come to a sense of what is most excellent, profound, admirable and true within human nature, but in a way that integrates or harmonises the different aspects of who we are as men (you cannot, for instance, ignore the biological drives of men in attempting to come to an integrated ideal of manhood.)
In short, it is wrong to see the natural man as being pre-moral and pre-social, and morality as being wholly conventional. I'll be interested to see how MacIntyre describes the unfolding of this sophist view of natural man later in his book.