Linear Destiny

Of course destiny is just a concept (or, if you are nominalist, just a word). I mean that there is no such thing as a destiny that our eyes can see, our tongue can taste, or our nose can smell. Destiny is a concept that implies that the word "future" is not only applicable to a specific verbal tense, but also to a state of affairs that "already exists in the present, namely when it should not exist by definition". If the future is something concrete it means that in the present there are seminal principles that somehow give reality to the future. For instance the genes of a newborn are supposed to cause the phenotypic traits of the adult: they are somehow the future in the present4. Briefly, what I want to show is that when speaking of destiny, the first issue we meet concerns the structure of causation. Which kind of causation are we speaking of in genetics?

Causation is the relation between two events, when there is one event which occurs, and produces, determines, or necessitates the other. The idea of causation seems to be reasonable and intuitive. Yet philosophers know that there is no way to demonstrate that in principle causes exist or, to put it differently, that each effect requires a cause5. Even the word "destiny" means nothing but a particular relation of causation. It comes from the Latin destinatio, which comes in turn from the verb destino, to fasten6. To destine thus literally means to fasten one thing to another. The idea underlying the word "destiny" is that events are fastened to each other as links of a chain. Genetics often makes use of this linear model of causation. According to a linear model of causation each event is a member of a series, being both the effect of a previous event and the cause of the next one. Causation can be consequently analysed as a linear sequence of discrete events. Of course the model can be complicated considering various concomitant sequences, that interact with each other and sometimes cross. In genetics, the simplest "complex model" provides for two concomitant sequences: genes and environment; other, and more complex, models consider neurodevelopment, societal influences, cultural heritage, and so on. The idea that causation is a matter of discrete events joined by links is clearly constitutive of Mendelian (unigenics) genetics7, but it remains as the landscape of multifactorial and polygenic genetics. Both the linear model of causation, and the image of the chain of destiny, come from Stoic philosophy (Von Harnim H, 1964). Stoics believed that destiny (r| st^apnEvri) was the rationale of the world, that it was the reason (Xoyoq) according to which the past was, the present is, and the future will be. Stoics thought that destiny was a chain of causes, namely an order and a connection that can never be forced or transgressed. No thing is without cause, and randomness does not exist in the universe.

Stoics have developed a complex doctrine about causes. They called "antecedent causes" (ama ou oux avsu), those causes that link events to each other in the causal chain8. Crysippos derives the existence of antecedent causes from logic. If something existed without causes—he argued—each proposition cannot be either true or false, since what lacks causes will be neither true nor false. But each proposition is true or false, thus there is nothing without causes. The problem of foreseeing was also central in Stoic philosophy. In fact any linear model of causation implies that one should be able to infer, or to foresee, from the existence of one event of the series, the existence of the others. Here, time is the crucial issue. Is a temporal relationship between the events required? From a trivial point of view the answer should be yes. Yet in a fully deterministic universe time's arrow is two-directional9. Any statement is simply true or false, no matter if it pertains to the past, the present, or the future. Nevertheless this position implied the existence of necessary futures, a thing that was denied by Stoicism. The issue of necessary futures is particularly intriguing. Stoics were definitely determinist, but they rejected the idea of necessary futures. Actually, the idea of necessary futures belongs to a naive determinism, very far from the sophisticated Stoic philosophy. According to Stoics, the future is not logically necessary since it always derives from conditional syllogisms, thus it is the result of various possibilities. That position created some interesting problems, in particular considering divination, a practice that was accepted by Stoic philosophers. Crysippos, one of the most influential Stoic philosophers, suggested that any kind of "true" divination should have the structure of a negative conditional syllogism (tollendo tollens), that means that any anticipation can be considered necessary only a posteriori, that is after its realisation10. This elegant solution is particularly intriguing since it corresponds, in the last analysis, to the solution given to modern geneticists in response to the challenge posed by genetic fatalism (Mordini E., 1997b).

The Stoic conception of destiny penetrated Roman culture and remained a part of the Latin culture of any educated persons during the Middle Ages and the Renaissance. The linear model of causation was adopted by modern scientists after the crisis of the Ar-istotelian-Thomistic model of causation and, through Galileo, Newton, Leibnitz, and Kant has survived till the present. At any rate, discussing the history of this model lies outside of the purpose of this article. What I would like to emphasise is that the idea of linear causation, which underlies several aspects of modern genetics, is not at all new, but it comes from a specific philosophic perspective (Stoicism). This is my first argument.

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