In this short quote below, J. Budziszewski explains just what virtue is:
If I said, “The excellence of a knife is its sharpness,” you would know what I meant: sharpness is the specific quality that enables a knife not just to cut, but to cut excellently. Another word for excellence is virtue. So you would also know what I meant if I said the virtue of an eye is clearness or that the virtue of a racehorse is swiftness. Clearness is what enables an eye to see excellently, swiftness is what enables a racehorse to race excellently. Once again we have a formula. A virtue or excellence of a thing (there may be more than one) is the specific quality that enables it to perform its function or proper work excellently and so achieve its highest good.Virtue is lost in our society because we no longer look at the entire person and his purpose for existence. Modern culture has elevated feeling above reason; the action or critique is wrong because it makes someone feel bad or it stifles his desire. Instead, one must understand what it means to be an excellent human being, which encompasses both intellectual and moral excellence. To not approach human excellence holistically turns tolerance into a vice, not a virtue.
Like our last formula, this one too can be applied to mankind. How do we know whether a particular human quality, such as courage or ruthlessness, is a virtue or not? The proper work of a human soul is using and following reason. So the quality is a virtue only if it helps it to do so excellently. For instance, one virtue is theoretical wisdom, the discipline of mind which helps us reason our way to truth while avoiding error; another is practical wisdom, the discipline of mind which helps us reason our way to good choices while avoiding evil.
But here we run into a problem. It looks at first as though the only true virtues are intellectual ones. What about moral virtues, such as courage, justice, self-control and friendliness? Isn't there a place for them? Yes, for two main reasons. To understand the first one, remember that more than one thing is active in the human soul: not only the power of reasoning itself but also the power of feeling and the power of desiring. … Just as the intellectual virtues discipline the thoughts, the moral virtues discipline the feelings and desires. An example of moral virtue that disciplines the feelings is courage, whereas an example of moral virtue that disciplines the desires is self-control.
To sum up, the main reason the intellectual virtues are not the only virtues is that different virtues are needed to put each of the different powers of the soul in rational order. The second main reason is that different virtues bring the soul into rational order in different respects.1