Coined by psychologist Kurt Lewin (1922), the term genidentity occurs in mereological inquiries into the notion of identity through time.

Lewin was interested in using the term to draw out similarities and differences in the way in which the sciences (in particular, biology and physics) treat the attribution of 'objecthood' or 'thingness' in what they study.

Starting from the skeptical viewpoint that we have no rationale for persistent objects, but we're just presented with a series of similar 'object-instances' from moment to moment, rather like the frames in a movie, Lewin claimed that:

when each of these object-instances is the genesis of its successor, then the "existential relation" of genidentity holds between all the object-instances in the series1
and so we are justified in talking of the persistence through time of one `thing' ("up to genidentity", as the mathematicians would say). Lewin's way of putting it was that any member of the series is genidentical with all the others.

Philosophical orthodoxy is that two or more things are identical if (and only if) all properties shared by one are shared by the other (Leibniz's Law). But on this view the question of identity through time can seem problematic. Are we to regard 'existing at time t' as a property? If so, can this property be true of an object which has the property 'existing at time t+1'? If two objects sharing all other properties are at positions p and q at the same instant, we cannot regard them as identical, but for two otherwise property-sharing objects existing at different times, ascription of identity is not ruled out.

In practice, we are willing to grant persistence even to objects which change their properties, sometimes grossly, over time. The biological notion of the organism is a classic example of this. We're quite happy to accept a balding 18 year old cat is the same animal as the fluffy kitten we found in a basket 18 years ago, just as we'd go along with the idea that it's still sleeping in the same basket.

This naive (or uncodified) notion of identity - 'the same so-and-so' - permeates our natural use of language - we take it for granted that the referents of nouns, apart from special cases like instantons, have duration and are sufficiently distinguishable from the background that the notion of identity is applicable, but sometimes the kind of distinction that the term genidentity aims at can be practically useful.

Consider a powerful flashlight used to move a spot of light at greater than lightspeed across the surface of the moon. To preserve relativity, we need to be sure that this isn't an example of a "thing" moving faster than light. Since each spot-instance is not the genesis of its successor instances - the flashlight is the generator of the spot - we can say that the members of the series are not genidentical and that therefore the spot is not properly a "thing", and the phenomenon does not contradict relativity. (Of course, this isn't the only way to resolve the question! It's just an example of the kind of argument where our term has found use.)

But if we take this step, of privileging a notion of identity let's say, we must consider the countless conundrums and corner cases that infest the whole business of mereology and mereological inquiry (if I may use the word "whole"). What about amoeba, for example? Are they not the genesis of both their "descendants", after fission? Is there therefore but one amoeba, consisting of billions of separate cell bodies? What, exactly, are our criteria for being the genesis of? Causal sufficiency? In that case, no organisms need apply, since they're all dependent on their environment. And if we try to provide some more refined class of causal relation, what is our notion of "causal", exactly? It's not the most notoriously perspicuous term, after all.

At the other end of the spectrum, we might embrace extreme relativism about identity and wholeness, claiming that whatever abstraction or superfluity we wish to make into a "thing", there will always be some rational schema that will allow it to appear as a term, and that is as much thingness as anything ever has anyway. But such skeptics of identity are reminiscent of those skeptics of causality of whom Hume tells us that after their meeting, they left by the door rather than the upstairs window. The thoroughgoing ascription of such arbitrariness to all questions of identity is not a practical proposition where, in practice, we are convinced of the reality of certain forms of identity, such as the identity of persons through time.

Perhaps the proper place of genidentity is as one of a set of criteria of identity which we can employ where and when they are useful, rather than as a panacea for all mereological ills. Nonetheless, wholeness is undeniably a philosophically problematic feature of our perceptions and thought, and the notion of genidentity is a brave attempt to address it.

1. Lewin's idea, but my words.

Lewin's formulation was published as:
Kurt Lewin, 1922: Habilitationsschrift, Der Begriff der Genese in Physik, Biologie und Entwicklungsgeschichte ("The Concept of Genesis in Physics, Biology, and Evolutionary History")

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