The phrase ‘tacit knowledge’ is normally connected with the work of the Hungarian-British scientist and philosopher Michael Polanyi (1888–1976). Polanyi’s (Reference Polanyi1958) slogan in philosophy was ‘We know more than we can tell’; ‘tacit knowledge’ was Polanyi’s name for all the things that we know but cannot tell.

Polanyi’s (Reference Polanyi1958) own examples of tacit knowledge include many cases of what philosophers often now call practical knowledge: knowing how to ride a bike, for instance, or the way that an apprentice carpenter may pick up good technique for making dovetail joints not by asking the master carpenter to explain them to her, but simply by watching and copying. In cases like these, not being able to explain in words what we want to teach isn’t necessarily just our inarticulacy. What is to be taught is a skill or a craft or a disposition, and these things involve a kind of understanding that we might say ‘outstrips words’ – or, we might also say, never gets as far as words. I mean the kind of understanding that is involved, for instance, in seeing how to reconcile with each other the demands of two different roles both of which I occupy: phronesis, or ‘judgement’, as philosophers often call it (cp. Chapter 1).

It is also very often true that learning a craft or skill involves learning to follow a rule. And while there can be rules about how to follow rules (lawyers often formulate this kind of meta-rule), there can’t be an infinite regress of rules about how to follow rules about how to follow rules about … At some point we just have to get on and follow the rule without any further guidance. As Ludwig Wittgenstein (again) puts it in the late notebook that has been published as On Certainty (Reference Wittgenstein1969, paragraph 219), at some point we have to ‘obey the rule blindly’. Doing this successfully involves another kind of tacit knowledge. It also, and connectedly, involves a disposition that we may call spontaneity.

Spontaneity means just doing what comes naturally. Provided what comes naturally to us is good, spontaneity is a good disposition for us to have (that is: it is a virtue). We aren’t always spontaneous, nor should we be, but sometimes we should. Most of us have a reasonable knowledge of when it is a good idea to act in a pre-planned manner, and when it is a good idea to be spontaneous. But we can’t be spontaneous by planning to be spontaneous, or by giving ourselves the command: ‘Be spontaneous!’ Being spontaneous by explicit plan, or in response to an explicit command, is not being spontaneous at all. So although we know when to be spontaneous, we don’t normally make that knowledge explicit in our own practical reasoning. And we not only don’t make it explicit; we can’t make it explicit, because making it explicit would be self-defeating. If we made our knowledge of when to be spontaneous explicit in our practical reasoning, then spontaneity would become impossible for us – which it clearly isn’t. So at least some of our knowledge of when to be spontaneous, and when not to be, has got to be tacit knowledge.

Spontaneity isn’t the only valuable disposition, or virtue, to which something like this pattern of reasoning applies. For instance, it is generally a virtue for someone to be unselfconscious; maybe, too, unselfconsciousness is involved in quite a range of other virtues. (So, for instance, Aristotle connects it with generosity: ‘It is the mark of the generous man that he does not consider himself’, Nicomachean Ethics 1120b5–6.) But first, I cannot self-consciously cultivate my own virtue of unselfconsciousness. No doubt there are some ways for us to cultivate unselfconsciousness, and it would be interesting to think about what they might be. But whatever they are, they can’t be direct and self-conscious methods, because self-conscious efforts to be unselfconscious are plainly and necessarily self-defeating.

Secondly, there are cases where what is needed is not the virtue of unselfconsciousness, but a different virtue: self-awareness or self-examination. For sure, good people have an understanding of which of these alternatives is appropriate in any particular situation. But as with spontaneity: whatever that understanding involves, it can’t involve nothing but explicit knowledge. Because then it would involve the self-conscious thought ‘Now I should be unselfconscious’ – and that thought too involves a kind of self-defeat.

So unselfconsciousness is a second virtue, alongside spontaneity, that involves necessarily tacit knowledge. A third case is humility. Consider the question ‘Will a genuinely humble person know that she is humble?’ Some philosophers, for example Julia Driver (Reference Driver1989), have argued that the answer to this question has to be ‘No’. If someone humble comes to believe that she is humble, then just by coming to believe that, she ceases to be humble. As Dickens’ famously odious character Uriah Heep seems scripted to bring out, to think or say ‘I am humble’ is itself an act of pride. So that being humble is not something that I can know. For as soon as I believe it, my belief isn’t true, and therefore isn’t knowledge.

For my own part, I wouldn’t go as far as Driver. I would want to distinguish the humble person in a reflective moment from the humble person in action. I agree that it is antithetical to humility to be thinking about how humble you are while acting humbly; that is what is wrong with Uriah Heep. But when you are not acting but sitting down and reflecting on your own character, I don’t think that it is impossible for you to think, and think truly and aptly, ‘Well, I may not have all the virtues, but at any rate I am humble.’ Indeed, in a moment of reflection like that, you might also think truly (and aptly), ‘I am unselfconscious’ and ‘I am spontaneous’. True, there would be something objectionably self-absorbed about someone who spent a lot of time reflecting on their own virtues in this way. But provided you don’t get too self-absorbed, there is nothing impossible about thinking such things offline. The only thing that is impossible is to act humbly – or unselfconsciously or spontaneously – with your attention firmly fixed on your own virtues.

I say that spontaneity, unselfconsciousness, and humility are three examples of important valuable dispositions, virtues, that crucially involve tacit knowledge. And I say that friendship involves tacit knowledge too. Why and how?

Well, friendship involves tacit knowledge because (as should be obvious from the discussion above) friendship involves spontaneity, unselfconsciousness, and humility – so friendship inherits their commitments to tacit knowledge.

So friendship involves tacit knowledge because a lot of the time, being a good friend to someone involves not thinking directly and explicitly about what is involved in being a good friend to them. Suppose my friend Jane comes round to my house late at night crying her eyes out. What do I do? At once I put the kettle on, sit her down, hold her hand, and wait for her to be ready to tell me what’s the matter. Do I act on a rule when I do this? Well, I might act in conformity with a rule, but that is a matter of correlation, not causation; it isn’t the rule that makes me act like this. For I don’t have to work out or deduce that this is the right thing to do. I don’t need to do a little sum of practical reasoning in my head: ‘Let me see, now – Jane is upset; I am Jane’s friend; I should do what a friend will do for Jane when she is upset, and that is …’ I just do it, without any conscious reasoning or deliberating at all; I just know that it’s the right thing to do. And this knowledge is tacit knowledge, which I don’t have to make explicit to know what is right to do, and to do it. What’s more, I would be much less of a friend to Jane if I did have to work it out. The more I have to reason out what friendship involves, the less I seem to understand what friendship involves. Such reasoning looks like a bad case of what philosophers sometimes call, following Bernard Williams (Reference Williams1981, 18), ‘a thought too many’.

Friendship, then, involves tacit knowledge both in its own right, and also because friendship involves the virtues of spontaneity, unselfconsciousness, and humility, and inherits their involvement with tacit knowledge. So if philosophers want to write well about friendship, they will have to do justice to the role of tacit knowledge in it.