Full Idea
To think of several virtues (as one unavoidably does) is nothing other than to think of the various moral objects to which the will is led by the one principle of virtue.
Gist of Idea
There is one principle of virtues; the virtues are distinguished by their objects
Source
Immanuel Kant (Metaphysics of Morals II:Doctrine of Virtue [1797], 406 Intro XIII)
Book Reference
Kant,Immanuel: 'The Metaphysics of Morals', ed/tr. Gregor,Mary [CUP 1991], p.207
A Reaction
So Kant commits to the Greek ideal of the unity of virtue - but not for Greek reasons. The unity of duty is what concerns Kant.
Related Idea
Idea 21417 How do we distinguish a mean? The extremes can involve quite different maxims [Kant]