G A Synchronic Regress of Explanations

We've seen that Aquinas elsewhere accepts the possibility of a diachronically infinite regress of explanations in answer to repeated applications of Ql. Here he is denying the possibility of a synchronically infinite regress of explanations in answer to repeated applications of Q2. Aquinas thinks that a causally linked series of efficient causes does not admit of an infinite regress just in case, for each cause in the series, its causally operating is required for its immediate successor's causally operating, so that the effect is not achieved unless all the causes in the series are operating simultaneously: 'in connection with efficient causes a regress that is infinite essentially {per se) is impossible—if, that is, the causes that are essentially required for some effect were infinitely many. For example, if a stone were moved by a stick, the stick by a hand, and so on ad infinitum' (ST la.46.2, ad 7). If in asking question Ql about S we picture a horizontal series of generating causes, stretching end p. 104


© Copyright Oxford University Press, 2006. All Rights Reserved back infinitely into the past, then the series of sustaining causes we're considering now in asking question Q2 about Sn should be pictured as vertical, the series of causes all of which must be operating at once, right now, in order to explain the present existing of anything that is 'on its own . . . related indifferently to . . . existing and not existing'.

Aquinas says that the impossibility he's alluding to here 'was proved above on the basis of Aristotle's reasoning' (lines 7-8). At this point in SCG, anything 'proved above' has to have been proved in chapter 13, and chapter 13 does contain not just one, but four, Aristotelian arguments against infinite causal regresses—three as sub-arguments in argument G1 and one in G3. But only one of those four, the third one in Gl, is clearly relevant to our case here: 'That which is moved instrumentally cannot move anything unless there is something that moves it initially (principaliter). But if one goes on ad infinitum as regards movers and things moved, all of them will be moving instrumentally, so to speak, because they are posited as moved movers; but nothing will be [operating] as the initial mover. Therefore, nothing will be moved' (13.95).

Like the Aristotelian example of the hand, the stick, and the stone, this argument has to do with causes of motion, rather than with sustaining causes as such. But the relevant sort of causes of motion, considered just as such, obviously is a species of sustaining cause: the stone stops moving as soon as the stick stops moving, and the stick stops moving as soon as the hand stops moving. This sub-argument from Gl insists that in such a synchronic causal series all the intermediate causes, however many there may be, must be merely instrumental, dependent for their causal operation on the causally prior, but temporally simultaneous, operation of some cause that is causally first in that series. So this inferred first cause cannot itself be an instrumental cause in the series, but must instead be the originally operative cause relative to which all the others in the causal series are instrumental. Aquinas does not, and need not, concern himself with how many intermediate instrumental causes may be involved in explaining a dependent being's presently existing. When he says that 'one cannot go on ad infinitum' in such a series, he means that it must be traceable to a first (or ultimate) cause, even if the causal distance between the first cause and the sustaining of the dependent being were infinitely divisible into simultaneously operating intermediaries.

end p.105

Kretzmann, Norman , (deceased) formerly Susan Linn Sage Professor Emeritus of Philosophy, Cornell University, New York

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