One man, one art.
This rule is laid down in the Republic of Plato as indispensable to the true city. If we contextualize this principle, which in the course of the Republic comes to light as the definition of justice (viz., "One man, one art"), we see that Plato defines justice as the principle which ensures maximal flourishing (in the sense of completion of a work) of the whole in which it inheres. This definition is justified by the agreement that justice is the human good, combined with the understanding that humans are creatures whose good consists in the completion of some work.
Justice, if the earlier treatment of it in the Republic as one art alongside others retains its currency, turns out to be the art of success in arts. But this definition renders it impossible to apply. One would have to have two arts, the art of justice and the art whose success it is to ensure. But the former art would consist in having only one art, which would make it impossible to have a second art to which it could apply.
There can therefore be no art concerning the accomplishment of the human good, and that is just what Aristotle thinks.