In Plato's Apology of Socrates, Socrates talks about having a daimon, a divine being/voice that tells him of things not to do. For Aquinas, what would this be?
The Greek philosophers' "daimons" were similar to angels (incorporeal intellectual beings).
Benedict Ashley, O.P., The Way toward Wisdom: An Interdisciplinary and Intercultural Introduction to Metaphysics, ch. 6, § D.2.a fn. 38:
Plato, Apology (c. 399/90 BCE), 33c:You have often heard me speak of an oracle or sign which comes to me, and is the divinity which Meletus ridicules in the indictment. This sign I have had ever since I was a child. The sign is a voice which comes to me and always forbids me to do something which I am going to do, but never commands me to do anything, and this is what stands in the way of my being a politician.Some take this to be simply the “voice of conscience”; but Socrates believed in the oracle of Delphi and often speaks of spiritual beings.
A "voice that tells him of things not to do" sounds half like moral conscience / synderesis:
"synderesis" is said to incite to good, and to murmur at evil, inasmuch as through first principles we proceed to discover, and judge of what we have discovered.
This question risks deletion because it juxtaposes Socrates and Aquinas on a topic Aquinas does not discuss and could not have discussed - Socrates' daimon. The question can be answered, however, as Geremia has shown. The point is what concept, if any, in Aquinas stands as a counterpart, if only incompletely, to the Socratic daimon? We can surely answer this question functionally regardless of what Aquinas knew of Socrates.
The leap from Aquinas back to the Ancient Greeks is perhaps not so great when we remember Aquinas’ indebtedness to Aristotle. Aquinas’s relation to Socrates, or (we had better say) the Platonic Socrates, is a different matter since so far as we know Aquinas was not acquainted in detail with the Platonic dialogues. His knowledge of Plato derived at least principally from the accounts of Plato’s writings and ideas in Aristotle’s Metaphysics and Nicomachean Ethics and in other Aristotelian works on which he produced commentaries.
If we are to look in Aquinas for a counterpart, however incomplete, to Socrates’ daimon or daimonion it is noteworthy that the daimon is referred to not only in the Apology (31D, 40AB) but in Euthyphro (3B), Alcibiades I (103A, 105A), Euthydemus (272E), Republic (496C), Phaedrus (242BC) and Theaetetus (151A). In Apology (31D)it is called thein, divine or spiritual (cf. Phaedrus, 242C).
It is a voice (Apol. 31D, Phaedrus 242C) often heard (to eithos semion: Euthydemus 272E, Phaedrus 242B). It never urges Socrates to positive action but admonishes him to avoid bad actions; it constrains him from doing what is wrong. It is prohibitive and is heard when Socrates is about to do something (Phaedrus, 242BC). In the same passage in Phaedrus Socrates adds that his ‘voice’ (‘which I thought I heard’) forbids him to leave ‘this very spot until I had made atonement [or ‘purified myself’] for some offense against the gods’. Nowhere else does the daimon issue such a prohibition.
For the record, Xenophon’s Socrates has a rather different ‘voice’ or ‘sign’, one that did not limit itself to prohibitions but instructed Socrates both as to what he should do and what he should not do (Xenophon, Apology 12-13; Memorabilia 1.1.2-9, 4.3.12, 4.8.5). It also enabled Socrates to give good advice to his friends. Those who took Socrates’ advice benefited from it; those who refused it came to regret doing so (Memorabilia, 1.1.4).
Compared with the Platonic account this seems a crudification of the daimon. What, then, in Aquinas might stand as a counterpart, however partial, to the Socratic daimon as related by Plato? The clearest candidate, as it seems to me, is conscientia. How so?
Aquinas makes synderesis knowledge of the basic principles of right conduct, conscientia the application (or misapplication) of knowledge of any kind, including synderesis, to particular acts, either in telling whether they have occurred or are occurring, or in judging whether they are right (P.A. Clarke).
Conscientia (conscience) matches the Socratic daimon (1) in that it is concerned with the rightness or wrongness of individual particular actions rather than with general or universal principles (of virtue or justice). There is also commonality in that (2) it comes unbidden. Socrates does not invoke his daimon and Aquinas believes that, special cases aside, the operation of conscience is automatic.
The major difference appears to be that (3) there is no question or suggestion that the Socratic daimon could be mistaken. Aquinas in contrast acknowledges both a right and an erroneous conscientia. Both types are equally binding as long as they are ‘certain’, i.e. genuinely believed on due reflection to be correct:
Clearly a right conscience binds. But what about a mistaken conscience? What of the man whose conscience tells him to do something - say, kill all unbelievers - that is, as a matter of fact, against the moral law. Nowadays we distinguish readily enough between inculpable and objectively right action, but refinements like this are only won at cost of much discussion. St Thomas's predecessors can be divided according to which of the three possible views they canvassed - that an erroneous conscience never, sometimes or always binds. That last was St Thomas's view and it has prevailed. Its paradoxical character is best seen from St Thomas's own examples, cited by Fr D'Arcy; the man who believes that he would sin by abstaining from fornication sins unless he fornicates; the man who believes that it would be sinful to embrace the Catholic Faith sins in becoming a Catholic. A certain conscience, right or wrong, always binds (M. B. Crowe).
As far as concerns virtue and erroneous conscience, Aquinas denies virtue to an action done at the behest of a certain, albeit mistaken, conscience … he inclines to say rather cautiously that it is 'not bad’ (M.B. Crowe).
So while the Socratic daimon finds a counterpart in Aquinas’ conscientia in two respects, in a third (that of error) the correspondence fails.
References Translations are from:
Plato: The Complete Works, ed. J. Cooper, Hackett, 1997.
P. A. Clarke, ‘Reviewed Work(s): Conscience in Medieval Philosophy. by Timothy C. Potts’, Mind , Jan., 1983, New Series, Vol. 92, No. 365 (Jan., 1983), pp. 128-129: 128.
M. B. Crowe, ‘Reviewed Work(s): Conscience and Its Right to Freedom by Erie D'Arcy’, Studies: An Irish Quarterly Review, Vol. 51, No. 202 (Summer, 1962), pp. 333-336: 334-335.
John M. Rist, ‘Plotinus and the "Daimonion" of Socrates’, Phoenix , Spring, 1963, Vol. 17, No. 1 (Spring, 1963), pp. 13-24: 15-17.