This has in its time borne a good many different meanings.
1. Koinos is the Greek for 'common', and we have already seen that sensus can be used as a translation of the Greek nous. Koinos Nous is defined by Epictetus thus: 'There are some things which undistorted men perceived by their use of their common faculties. This state of affairs is called Koinos nous' (III, vi, 80).i Here we have, almost exactly, what common sense often means; the elementary mental outfit of the normal man. Communis sensus would be a very natural way of turning Koinos nous into Latin, but clear examples of communis sensus to mean intelligence are not very easy to find. This, from Phaedrus, is, I think, certain. The Fox, finding a tragic mask, remarks, after sniffing and trundling it, 'What a fine physiognomy to have no brain inside it!' The moral applies, says Phaedrus, to those people who have office and fame but no sensus communis (I, vii).
2. Distinct from this, so far as I can see, is the use of communis sensus as the name of a social virtue. Communis (open, unbarred, to be shared) can mean friendly, affable, sympathetic. Hence communis sensus is the quality of the 'good mixer', courtesy, clubbableness, even fellow-feeling. Quintillian says it is better to send a boy to school than to have a private tutor for him at home; for if he is kept away from the herd (congressus) how will he ever learn that sensus which we call communis? (I, ii, 20) On the lowest level it means tact. In Horace the man who talks to you when you obviously don't want to talk lacks communis sensus.1 To say 'lacks common sense' would be a mistranslation. But the fact that the mistake is so tempting and the alteration so comparatively slight shows that these two semantic regions have at least a strip of common frontier. In that way even this usage may have made some small contribution to the later meaning.
3. Quite distinct from these is communis sensus or 'common wit' as a technical term in medieval psychology; originally, I presume, a rendering of Greek Koine aisthesis.2 The old psychologists gave man five 'outward' and five 'inward' wits (or sense). The five outward wits are what we call the five senses today. Sometimes they are called simply the wits; hence in Shakespeare 'my five wits nor my five senses'.3 Which five you lose, or whether you lose all ten, when you are frightened 'out of your wits' or 'out of your senses' I don't know; probably the inward ones.
The five inward wits were originally memory, estimation, fancy, imagination, and common wit (or common sense). By Burton's time the list has been reduced to three,4 but common sense is still one of them, and his account of it will serve out turn: it is 'the judge or moderator of the rest ... by whome we discern all differences of objects; for by mine eye I do not know that I see, or by my ear that I hear, but by common sense, who judgeth of sounds and colours; they [sc. the eye and ear] are but the organs to bring the species (appearances, sense-data) to be censured (judged)'. It is in fact something like apperception; it turns mere sensation into coherent experience. We see its function in the 1590 Arcadia (III, xviii, 9) when Sidney explains how two combatants could go on fighting despite their severe wounds --- 'Wrath and Courage barring the common sense from bringing any message of their case to the minde.'
It will be noticed that a man in whom the common sense or wit is suspended is not entirely in his right mind. One in whom it was permanently lacking would be an imbecile. Here we have yet another semantic pressure which could help common sense towards the meaning 'gumption'.
4. Sensus, as we have seen, means all the erlebt; our experience, emotions, thoughts, apprehensions, and opininons. The communis sensus of mankind is what all men have 'been through' (e.g. pain and pleasure) or feel emotionally (fears and hopes) or think (that half a loaf is better than no bread) or agree to be true (that two and two make four).
Now the word common is here ambivalent.
(a) It may contrast the sensus of the huamn race in general, unfavorably, with what experts think and know or what choice spirits apprehend and feel. Common, taken that way, is 'common or garden', nothing above the ordinary; if you like, vulgar.
Thus Cicero says that in all the arts except one (oratory) that is best which is furthest from the sensus of the ignorant, but in public speaking you have to stick to the common mode of spech and the custom of the communis sensus.5 You are not addressing men of learning or fine feeling; you can use only what will 'find an echo in every bosom'. In Love's Labour's Lost the 'godlike recompense' of study or learning is to know 'things hid and barr'd from common sense' (I, i, 55-7), things beyond the thought and apprehension of ordinary men. When Spenser says that the pains of lovers seem ' 'gainst common sense, to them most sweete'6 he does not mean, as we should if we used the same words, that the lovers are fools who like their pains contrary to all reason. He means that the gentle heart finds somehow sweet what the 'swainish and ungentle breast' with its merely 'common' apprehension would find disagreeable.
(b) But common may also contrast the sensus of humanity in general, favorably, with what is thought or felt by the irrational, the depraved, the sub-human. Common, so taken has no association with vulgar. It is the quod semper, quod ubique, the normal and indeed the norm.
It is this, though he happens not to use the words common sense that Hooker is thinking of when he says that 'the general and perpetual voice of man is as the sentence of God himself' (I, viii, 3). So is Cicero, when he says that some principle is vouched for 'by truth and the nature of things and the sensus of every man7 Seneca is particularly illuminating. He first produces philosophical auuthority to show that the wise man is self-sufficient. But then he confirms it8 from a passage out of a comic poet in order to show that the sensus (plural) are communes, are 'universal convictions'. The 'common sense' or vote or sentence of humanity is august enough to confirm even the teaching of the Stoics. St Augustine speaks of people 'divorced by some madness from the communis sensus of man'.9 Centuries later, the Jesuit Mariana writes that communis sensus 'is, as it were, the voice of Nature whereby we may discern good from evil.'10
Thus the ambivalence of the word brings it about that one's sense may be disparaged by that adjective, but equally one's sense may be all the better for its 'commonness'.
1 Satires I, iii, 66.
2 Aristotle, De Memoria, 450a.
3 Sonnet CXLI
4 See Pt. I, Sec. I, Mem. 2, Subs. vii.
5 De Oratore I, iii, 12
6 F.Q. IV, x, 2.
7 De Finibus IV, 19.
8 Epistle 9
9 The Two Souls, 10.
10 De Rege I, vi, 1598
Also obviously relevant is the entry for "Common Sense" in Oxford English Dictionary