Tahafut_al-Tahafut-transl-Engl-van-den-Bergh

may assume that this cause has a cause and so ad infinitum. We do not
therefore arrive at an existent without cause-for this is the meaning of the
expression `entity of a necessary existence'-unless by the possible which
Avicenna assumes as the opposite of what has no cause we understand
the truly possible, for in these possibles there cannot exist an infinite
series of causes. But if by `possible' is meant those necessary things
which have a cause, it has not yet been proved that their infinite number is
impossible, in the way it is evident of the truly possible existents, and it is
not yet proved that there is a necessary existent which needs a cause, so
that from this assumption one can arrive at a necessary entity existing
without a cause. Indeed, one has to prove that what applies to the total
causal series of possible entities applies also to the total causal series of
necessary existents.
Ghazali says:
The terms `possible' and `necessary' are obscure, unless
one understands by `necessary' that which has no cause for
its existence and by `possible' that which has a cause for its
existence;' then, by applying the terms as defined to the
statement, we say: Each member of a causal series is
possible in this sense of `possible', namely, that it has a
cause additional to its essence, but the series as a whole is
not possible in this sense of `possible'.'' And if anything else
is meant by `possible', it is obscure. If it is objected that this
makes the necessary existent consist of possible existents
and this is impossible, we answer: By defining `necessary'
and `possible' as we have done, you have all that is needed
and we do not concede that it is impossible. To say that it is
impossible would be like saying that it is impossible that
what is eternal should be made up of what is temporal, for
time according to you philosophers is eternal, but the
individual circular movements are temporal and have initial
terms, though collectively they have no initial term; therefore,
that which has no initial term consists of entities having initial
terms, and it is true of the single units that they have a
beginning, but not true of them collectively. In the same way
it can be said of each term of the causal series that it has a
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