(2) No agent can be perfectly virtuous without such determination;
(3) The constitution of the body, by affecting the details of the emotional and volitional life, fits us for specific types of virtues and vices and so determined particular details and idiosyncrasies of our moral characters.
Hayden Ramsay has a nice paper on Aristotle's and Aquinas's accounts of natural virtue ["Natural Virtue," Dialogue 37:2 (1998) 341-360] in which he notes that, despite the immense difference between their virtue theories, Aquinas and Hume agree on all three of the above points (Ramsay combines the first two into one thesis; I've just broken it into two). An example of a natural virtue for Aquinas would be the understanding of first principles (this is the sort of natural virtue that is common to all human beings), or the temperamental disposition of a particular man that allows him to understand certain things better than another (this is the sort of natural virtue that varies from individual to individual). Aquinas discusses the matter in ST 1-2.51.2).
Ramsay notes that the concept of natural virtue is important for natural law: natural virtue is what identifies the general ends of our nature; they are the seeds that are developed into virtue in the proper sense. This implies objective moral norms and the limits of consequentialism: "Acts which directly and intentionally threaten (anyone's) realization of some human good(s) are practically unreasonable, incompatible with perfect virtue, and so immoral even when performed for good ends" (Ramsay, p. 350).
When Hume talks about 'natural virtues' he has a different meaning, since many of Hume's natural virtues are acquired virtues; the contrast in Hume is not, as in Aquinas, between 'natural' and 'acquired', but between 'natural' and 'artificial', where the artificial virtues are the virtues that intrinsically presuppose a reference to public good. He notes this explicitly in a discussion of natural virtue in T3.3:
The only difference betwixt the natural virtues and justice lies in this, that the good, which results from the former, arises from every single act, and is the object of some natural passion: Whereas a single act of justice, consider'd in itself, may often be contrary to the public good; and `tis only the concurrence of mankind, in a general scheme or system of action, which is advantageous.
Both types are "intermixed" in our moral judgments. It's interesting that Hume, too, is not a consequentialist; I've discussed this point before.
