Whether the cause of a monstrosity stems from the matter or from the efficient cause.

Further one asks whether the cause of a monstrosity stems from the matter or from the efficient cause.

And it seems to stem from the efficient cause. For when completion proceeds from something, then incompletion comes from the same thing. For the Philosopher says that just as a completed effect comes from the agent's strength, so too does an incomplete effect come from its weakness. Therefore, the agent is the cause of monstrosity.

Moreover, nothing leads itself into act. Therefore, matter (whether it be superfluous or diminished) will never produce a monster unless it is led by another into act. Therefore, the agent will be the cause of monstrosity.

The Philosopher says the opposite.

One must say that the primary source of monstrosity is rooted in matter, and the secondary source of monstrosity comes from the efficient cause, because a natural agent acts on the matter supplied to it and acts for the sake of an end. If, then, it finds matter proportioned to the end, it produces the appropriate effect; if, however, it finds the matter either superfluous or diminished, it will produce an effect that is either superfluous or diminished.

Just as it is with a large number of children, so is it for a large number of members. For just as an agent, when it finds matter proportionate to a large number of children, produces many children, and when it finds that the matter is insufficient even for one it will cease to act, so it is for the generation of members, for example, the hands. When the formative power has completed the five fingers for the hand, if there is still sufficient matter for one more finger and nature is not idle, it will produce a sixth finger. Thus nature first intends only five fingers, but after it finds abundant matter, it intends a sixth. And it is the same when there is diminution, as when matter is lacking after four fingers have been produced.

And if one inquires: Why does nature not divide the matter of the sixth finger into the other five, so that each finger would receive a part of the matterfor this would be better than to produce sixone must reply that nature proceeds in an ordered way and does not produce just anything from just any matter, but produces a proportional thing from proportional matter. Thus, once nature has formed the principal parts, it then produces accompanying proportional parts. Thus it forms the fingers and increases their size according to the proportion of the remaining part of the hand and the arm, and this is why it does not send more matter to the fingers than the fingers demand. So if it were to divide the matter of the sixth finger into the other five, then it would make the five too big and exceed all proportion. And besides, nature works in stages. Thus five fingers are formed before the material for the sixth is distributed, and this is why that matter cannot be divided partially among the five fingers.

It is the same for other monsters. For in those who generate several children in the womb, each child is separated from another by an intervening membrane [ pellicula]. Thus if it happens that this membrane is ruptured above and not below, then the material for the two children comes together above but is kept apart below, and then there will be generated a human being (or a horse) having one body and one head but having the feet of two individuals. If, however, contrariwise, the membrane is ruptured below and remains intact above, then the fetus will be one below and it will be divided above, having two heads. And it happens the same way for other monsters. Thus a monster's first flaw stems from matter, but because matter does not lead itself into act, but is led into act by an agent, and for this reason the agent is therefore the secondary cause of the monstrosity. Nevertheless, if the matter and the agent are proportional, or diminished or superfluous, then the effect will be partially monstrous, because it will not be complete. But still monstrosity stems from matter, because although it is proportionate in se, it nevertheless is disproportionate compared to the agent, because matter and agent are not enough, but proportion is required as is the obedience of one to the other.

With this a response to the arguments is clear. For they prove that the agent is in some measure the cause of the monstrosity, and this we concede, and for this reason, etc.