G is for

Ortega wasn’t alone in understanding the dynamic of the generations as decisive in the historical dynamic. Later sociologists like Mannehiem and Feur also explored the importance of temporal dialectics under whatever name and of course, from a wide variety of ideological positions.

Back in the day (when I first encountered Silo’s teachings) you’d have found a definition in the materials of the time something like this: 1) Plotinus explained that the g is based on alteration. The alteration of a system is the result of an evolutionary process. When the entropy of a system increases (synthesis) its structure is altered and new relations arise, and therefore, so do new entities (differentiation). These are the progressive elements that will replace the whole previous structure, forming a new one. 2) The g determines the articulation of the historical change and the fundamentals of the historical process. The coexistence of several generations is what permits us to speak of an historical moment. The g is characterized by the situation of a group of individuals of the same age (see: temporal class). 3) At a given historical moment, the following generations exist: the g being born, the one in apprenticeship, the one in struggle against the generation in power, the oppressive g (the one in power), the displaced one, and that of the 'survivors'. Of all of these, only three are considered in a dynamic situation: the one in apprenticeship, the one that fights against oppression, and the oppressive g (the g in power). Those of the children, the old, and the 'survivors' appear as marginal generations, until the children enter apprenticeship and the others die. To Ortega, the g "is one and the same as the structure of human life at each moment". 4) However, that which allows a coetaneous group to be considered within the same g is their situation not only in relation to power in general, but also the similarity of their vital processes which is psychologically reflected in the similarity of the stage of the reverie nucleus. Indeed, a fundamental modification of the reverie nucleus corresponds to each vital stage: childhood, adolescence, youth, maturity, decline, and old age.
Siloism by H. Van Doren: Aconcagua Press, 1972

Here’s the definition that Silo left us with:
As social production develops, the human horizon expands, but the mere existence of social objects does not guarantee the continuity of this process. For New Humanism, continuity is a function of the interaction among human g which transforms them in the process of production. These g which promote continuity and development, are dynamic structures – they are social time in motion – without which a society would fall back into a state of nature and lose its condition of historical society, as occurred in the destructuring of the ancient empires.
Wars have been decisive factors in the “naturalization” of societies by destroying continuity through the violent decimation of the younger generation. Within a single temporal horizon, in a single historical moment, those who are contemporaries coincide, coexist, but do so from landscapes of formation that are specific to each generation by virtue of its difference in age from other g. This fact marks the enormous distance in perspective separating the g, which, though they occupy the same historical stage, do so from different situational and experiential “levels.” It also happens that in every historical time there coexist g of different temporal levels, with different retentions (memories) and protensions (or future plans), and which, therefore, form different situations. The bodies and behavior of children and the elderly reveal, for the active g the presence of something they come from and toward which they are headed, and, in turn, for the young and old extremes of that triple relation, temporal circumstances that are also extreme. But this never remains fixed, because as the active g grow old and the oldest g die, children are gradually transformed and begin to occupy active, central positions. And new births continually reconstitute society. When, as an abstraction, one “detains” this incessant flow, it is possible to speak of a “historical moment” in which all the members occupying the same social stage can be considered contemporaries, living in a single time (in the sense of datability). But these members observe a non-homogeneous coetaneousness (with respect to their internal temporality and experience). The g. most contiguous to the active g. strive to occupy the central activity (the social present), in accordance with their particular interests, establishing a dialectic relationship with the g in power in which we can observe the new surpassing the old.
The topic of the g has been treated by a number of authors, among whom Dromel, Lorenz, Petersen, Wechssler, Pinder, Drerup, Mannheim and, of course, Ortega y Gasset stand out.

A Dictionary of New Humanism, in Silo; Collected Works Vol 2.