The fallacy of composition is the fallacy of inferring from the fact that every part of a whole has a given property that the whole also has that property. This pattern of argument is the reverse of that of the fallacy of division. It is not always fallacious, but we must be cautious in making inferences of this form.
A clear case of the fallacy of composition is this:
(1) Every song on the album lasts less than an hour.
(2) The album lasts less than an hour.
Obviously, an album consisting of many short tracks may itself be very long.
Not all arguments of this form are fallacious, however. Whether or not they are depends on what property is involved. Some properties, such as lasting less than an hour, may be possessed by every part of something but not by the thing itself. Others, such as being bigger than a bus, must be possessed by the whole if possessed by each part.
One case where it is difficult to decide whether the fallacy of composition is committed concerns the cosmological argument for the existence of God. This argument takes the contingency of the universe (i.e. the alleged fact that the universe might not have come into being) as implying the existence of a God who brought it into being. The simplest way to argue for the contingency of the universe is to argue from the contingency of each of its parts, as follows:
(1) Everything in the universe is contingent (i.e. could possibly have failed to exist).
(2) The universe as a whole is contingent (i.e. could possibly have failed to exist.
It is clear that this argument has the form of the fallacy of composition; what is less clear is whether it really is fallacious. Must something composed of contingent parts itself be contingent? Or might it be that the universe is necessarily existent even though each of its parts is not?
Another controversial example concerns materialistic explanations of consciousness. Is consciousness just electrical activity in the brain, as mind-brain identity theorysuggests, or something more? Opponents of mind-brain identity theory sometimes argue as follows:
(1) The brain is composed of unconscious neurons.
(2) The brain itself is not conscious.
It is certainly difficult to see how consciousness can emerge from purely material processes, but the mere fact that each part of the brain is unconscious does not entail that the whole brain is the same.