Initially, Durkheim appears to rule out any attempt to justify or invalidate morality, even within a society. In Sociology and Philosophy, he cautiously states: Moral reality, like all reality, can be studied from two different points of view. One can seek to know and understand it; or one can propose to judge it. The first of these problems, which is theoretical, must necessarily precede the second, and it is the only one that will be dealt with here ¹.

The aim, then, is to understand morality, not to judge it; and yet Durkheim subtly shifts from one perspective to the other, as we shall see.

Durkheim's primary objective was to demonstrate that a moral fact constitutes a social fact. To this end, he relies on the idea that an act can never be called moral if its sole object is the interest of the individual or of other individuals. Morality can only have as its objective the group formed by a plurality of associated individuals—that is, society, understood as a personality qualitatively different from the individual personalities that make it up. It follows that Morality begins where attachment to any group begins ².

Above all, a moral fact is a social fact because it corresponds precisely to Durkheim's definition of a social fact in the Rules of Sociological Method. In that work, Durkheim set out to establish sociology as an autonomous discipline, distinct from and irreducible to the fields to which it had often been assimilated, such as psychology and biology.

At the time, the prevailing view held that, since society consisted of individuals, social rules originated in individual minds, making it possible to ground sociology in psychological analysis and thus deprive it of any independent standing in favour of psychology.
Similarly, society was frequently compared to a biological organism, with individuals likened to its cells. This biological metaphor tended to strip sociology of any distinct consistency, its content and methods assumed to mirror those of biology.

Durkheim counters this by demonstrating that a social fact has its own distinct definition, one that resists both biologistic and psychologising interpretations. He argues that even when individuals conform to social rules, aligning their personal beliefs with prevailing norms, those rules do not originate in the individual mind (i.e., they are not psychological); rather, they are externally imposed, independent of individual preference. These rules exist before us and outside us, endowed with an imperative and coercive power by virtue of which [they] impose themselves on us whether we like it or not. They cannot therefore be derived from my mind, but are imposed upon it—which is why sociology cannot be reduced to psychology.

This leads Durkheim to conclude that a fact is social by virtue of being obligatory: A social fact is recognisable by the power of external coercion which it exercises or is likely to exercise over individuals; and the presence of this power is in turn recognisable by the existence of some specific sanction or by the resistance which the fact opposes to any individual undertaking which tends to do it violence ³.

A moral fact is therefore essentially a social fact, placing it within the province of sociology by virtue of its obligatory nature. The question then arises whether sociology, as the science of social facts, can provide a foundation for morality as a social fact. In other words, does sociology offer a solution to the problem of grounding morality?
Durkheim's answer is affirmative: a moral rule will be justified if social causes have supported its development and continuity; another will be set aside if the social conditions that prompted its adoption no longer obtain, leaving it as a vestige of a bygone social state:
The awareness that society acquires of itself in and through opinion may be inadequate to the underlying reality. It may be that opinion is full of survivals, lagging behind the real state of society [or that] certain principles of existing morality are, for a time, rejected in the unconscious. Fortunately, the science of morality makes it possible to rectify these errors ⁴.

From this approach to grounding specific moral rules, Durkheim derives a basis for morality as a whole: It will be maintained that no morality can ever be desired other than that which is demanded by the social state of the time. To desire a morality other than that which is inherent in the nature of society is to reject it, and, consequently, to reject oneself ⁵.

1. Sociology and philosophy, chap. II
2. Ibid.
3. Rules of sociological method, chap. I
4. Sociology and philosophy, chap. II
5. Ibid.