Avicenna's argument for the Impossibility of Infinite Regress

A causal series is a chain of existents connected together by the relation of causality such that some members are the causes of others and certain others are the effects of others; now a causal series may either be definite and finite, or alternatively indefinite and infinite; a finite causal series is one which has a pure cause and a pure effect as its extremes, where a pure cause is an existent that is only a cause of all the other posterior members of the causal chain without itself being the effect of any; and a pure effect is an existent that is only an effect of all the prior members of the series without itself being the cause of any; besides a pure cause and a pure effect, a finite causal series may consist of several intermediate members, each of which is both a cause in relation to the member posterior to it, as well as an effect in relation to the one prior to it; now if we consider a definite or finite causal series consisting of only three members say a,b & c, where 'a' is a pure cause of both 'b' and 'c' such that it is the immediate cause of 'b' but the remote cause of 'c'; and 'c' is a pure effect of both 'a' and 'b' with the difference that 'a' is its remote and 'b' its proximate cause; so it would not be possible for 'b and 'c' to come into being without 'a'; because both the pure effect namely 'c', and the intermediate effect 'b' are essentially-contingent or possible in essence and therefore cannot exist unless their Existence is rendered necessary by a cause, which in this case is 'a'; now it makes no difference whether the intermediate members of the series denoted here by 'b', are finite or infinite in number, since they will retain the attribute of being an effect regardless of their finitude or infinity of their numbers; and since the intermediate member 'b' in the series cannot exist without a pure cause which itself is uncaused therefore an infinite causal series is impossible;

Now an infinite causal series is one, every member of which is an effect of its immediately prior or anterior members; therefore every member of such a causal series is essentially-contingent; now the series itself is a composite whole which depends on each of the individual members (every one of which is an effect) for its composition; and that which depends upon something contingent must itself also be contingent; and that which relies upon an effect for its existence must itself also be an effect of some cause; now this cause of the supposed infinite series cannot itself be a member of the series, as in that case it would also be the cause of its cause and consequently a cause of its own essence; but it is evident that nothing can be a cause of its own essence, since a thing cannot exist before it exists; hence the cause of this series must necessarily be extraneous to it, and therefore uncaused.