I am a great believer in the rather naïve notion that a philosopher’s morals and way of life ought to match the ideal he professes in his theory. Thus, I believe it perfectly legitimate to argue ad hominem from the mismatch between the philosopher’s life and his theory. As a matter of fact, there are some examples of this style of argument that I find particularly elegant and persuasive. The best example I can give is one by Lord Shaftesbury directed against Thomas Hobbes’ egoistic theory of morality.
It is fairly well-known that Hobbes espoused a theory in which human nature was portrayed as essentially self-interested, and that no-one did anything that wasn’t intended to further his own perceived interests. In essence, for Hobbes “man is wolf to man” and can only be kept on the straight and narrow if it is to his own private advantage to do so.
To this narrow view, Shaftesbury answered as follows: “Sir! The Philosophy you have condescended to reveal to us, is most extraordinary. We are beholden to you for your Instruction. But, pray, whence is this Zeal in our behalf? What are We to You? Are You our Father? Or if You were, why this Concern for Us? Is there then such a thing as natural Affection? If not; why all this Pains, why all this Danger on our account? Why not keep this Secret to Your-self? Of what advantage is it to You, to deliver us from the Cheat? The more are taken in it, the better…. ‘Tis not fit we shou’d know that by Nature we are all Wolves. Is it possible that one who has really discover’d himself such, shou’d take pains to communicate such a Discovery?” (Characteristicks of Men, Manners, Opinions, Times, Vol. I, p. 92).
If man is really wolf to man, why would a true wolf wish to undeceive his sheep? Would it not be more in Hobbes’ self-interest to keep this secret to himself? His need to share his discovery betrays the very sociability his arguments deny.
Shaftesbury borrowed this argument from Epictetus, who had leveled it against Epicurus. Epictetus, in Discourses, 2.20, said: “For what does he [Epicurus] say? ‘Be not deceived, men, nor led astray, nor mistaken; there is no natural fellowship with one another among rational beings; believe me. Those who say the contrary are deceiving you and leading you astray with false reasons.’ Why do you care, then? Allow us to be deceived. Will you fare any the worse, if all the rest of us are persuaded that we do not have a natural fellowship with one another, and that we ought by all means to guard it? Nay, your position will be much better and safer. Man, why do you worry about us, why keep vigil on our account, why light your lamp, why rise betimes, why write such big books?... Come, do you interest yourself in sheep because they allow themselves to be shorn by us, and milked, and finally to be butchered and cut up? Would it not be desirable if men could be charmed and bewitched into slumber by the Stoics and allow themselves to be shorn and milked by you and your kind [i.e. Epicureans]?” (There is a similar example in Discourses, 1.20.)
In his commentary on Hugo Grotius’ De Iure Belli ac Pacis, Jean Barbeyrac (1674-1744) refers to both Epictetus and Shaftesbury on this subject: “The late Lord Shaftesbury has reasoned in the same manner [as Epictetus], but with a lively Turn, which gives his Piece the Air of an Original, against Hobbes, who with still more Warmth than his Master Epicurus, undertook to persuade the World that all Men are by Nature so many Wolves one to another. See that Lord’s Essay on the Use of Raillery, &c. p. 64, & seq. printed at the Hague in the Year 1710” (Barbeyrac, in Hugo Grotius, The Rights of War and Peace, anonymous 1738 English edition, vol. I, p. xv, note).
A somewhat similar point had been made before Shaftesbury, as an explicit animadversion on Hobbes, by Richard Cumberland (1632-1718), in his De Legibus Naturae (1672; English translation, 1727), ch. V, §15: “The Understanding has a strong Natural Propension, to make itself Master of those things, which may be useful to others as well as to ourselves. Hence all the Sciences, which have been found out by great application of Mind, and made Publick for the Common Benefit, have taken their rise.” In other words, the practice of scientists, and their need to share their discoveries, demonstrates their natural sociability
The argument from self-refutation was deployed, with indifferent effect, by Plutarch against the Stoics. At the beginning of his essay “On Stoic Self-Contradictions” (in Moralia, Vol. XIII, Pt. II), he justified the practice of refuting an argument by appealing to the practice of one’s opponent as follows: “In the first place I require that the consistency of men’s doctrines be observed in their way of living, for it is even more necessary that the philosopher’s life be in accord with his theory than that the orator’s language, as Aeschines says, be identical with that of the law. The reason is that the philosopher’s theory is a law freely chosen for his own, — at least it is if they believe philosophy to be not a game of verbal ingenuity played for the sake of glory but, as it really is, an activity worthy of the utmost earnestness.”
As an argument form, appeal to practical self-refutation can be well done (e.g. Shaftesbury) or poorly done (e.g. Plutarch). But there is no reason why philosophers should consider it an illegitimate way of arguing, unless one’s conception of philosophy is so perverted as to preclude the belief that theory ought to bear a relation to practice.