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

I say:
cause, but not of the series as a whole. And so not
everything that is true of single units is true of their
collectivity, for it is true of each single unit that it is one and a
portion and a part, but not true of their collectivity; and any
place on the earth which we choose is illuminated by the sun
by day and is dark by night, and according to the
philosophers each unit has begun, but not the whole.
Through this it is proved that the man who admits temporal
entities without a beginning, namely, the forms of the four
elements,' cannot at the same time deny an infinity of
causes, and we conclude from this that because of this
difficulty there is no way in which they can prove the First
Principle, and their dichotomy is purely arbitrary.
The assumption of infinite possible causes implies the assumption of a
possible without an agent, but the assumption of infinite necessary entities
having causes implies only that what was assumed to have a cause has
none, and this argument is true with the restriction that the impossibility of
infinite entities which are of a possible nature does not involve the
impossibility of infinite necessary entities. If one wanted to give a
demonstrative form to the argument used by Avicenna one should say:
Possible existents must of necessity have causes which precede them,
and if these causes again are possible it follows that they have causes
and that there is an infinite regress; and if there is an infinite regress there
is no cause, and the possible will exist without a cause, and this is
impossible. Therefore the series must end in a necessary cause, and in
this case this necessary cause must be necessary through a cause or
without a cause, and if through a cause, this cause must have a cause
and so on infinitely; and if we have an infinite regress here, it follows that
what was assumed to have a cause has no cause, and this is impossible.
Therefore the series must end in a cause necessary without a cause, i.e.
necessary by itself, and this necessarily is the necessary existent. And
when these distinctions are indicated, the proof becomes valid . But if this
argument is given in the form in which Avicenna gives it, it is invalid for
many reasons, one of which is that the term `possible' used in it is an
equivocal one and that in this argument the primary dichotomy of all
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