Nicomachean Ethics

Virtue then is “a state apt to exercise deliberate choice, being in the relative mean, determined by reason, and[14] as the man of practical wisdom would determine.”

It is a middle state between too faulty ones, in the way of excess on one side and of defect on the other: and it is so moreover, because the faulty states on one side fall short of, and those on the other exceed, what is right, both in the case of the feelings and the actions; but Virtue finds, and when found adopts, the mean.

And so, viewing it in respect of its essence and definition, Virtue is a mean state; but in reference to the chief good and to excellence it is the highest state possible.

But it must not be supposed that every action or every feeling is capable of subsisting in this mean state, because some there are which are so named as immediately to convey the notion of badness, as malevolence, shamelessness, envy; or, to instance in actions, adultery, theft, homicide; for all these and suchlike are blamed because they are in themselves bad, not the having too much or too little of them.

In these then you never can go right, but must always be wrong: nor in such does the right or wrong depend on the selection of a proper person, time, or manner (take adultery for instance), but simply doing any one soever of those things is being wrong.

You might as well require that there should be determined a mean state, an excess and a defect in respect of acting unjustly, being cowardly, or giving up all control of the passions: for at this rate there will be of excess and defect a mean state; of excess, excess; and of defect, defect.

But just as of perfected self-mastery and courage there is no excess and defect, because the mean is in one point of view the highest possible state, so neither of those faulty states can you have a mean state, excess, or defect, but howsoever done they are wrong: you cannot, in short, have of excess and defect a mean state, nor of a mean state excess and defect.