Le coeur a ses raisons que le raison ne connaît point; on le sait en mille choses.
The heart has its reasons whereof reason knows nothing; one knows it in a thousand things.
It is sometimes difficult to convey how much modern people have a one-dimensional view of their own minds. Everything is jumbled and conflated together until we can hardly talk of 'belief' or 'intuition' or 'thought' or 'reason' without equivocating ourselves to high heaven. But it is possible to recognize that our thoughts have a texture to them, that they are not all the same kind of thing; and this pertains the maxim above, because Pascal is, in fact, arguing for precisely this sort of distinction.
I. Blaise Pascal
Blaise Pascal (1623-1662) was a seventeenth-century prodigy, a pioneer in projective geometry and probability theory. He was early on a participant in the Jansenist movement, but entered what is often known as his 'worldly period', in which he distanced himself from religious matters, from 1648-1654; this period was brought to an end by a brush with death in 1654, which led to an intensified interest in religious topics. He took up the cause of Jansenism against the Jesuits with his Provinical Letters, an attack on Jesuit casuistry. As he was finishing up the Letters, his 10-year-old niece Marguerite Périer, ceased to suffer from a condition thought to be incurable when a relic, said to be a thorn from Christ's crown of thorns, was touched to her eye; this event, the Miracle of the Holy Thorn, took the already growing Jansenist movement and galvanized it, sparking it to new life. It was in this context that Pascal set out to write his life's masterwork, an Apologie de la religion Chrétienne (Defense of the Christian Religion). By the time of his death, however, the work still consisted only of scattered notes and fragments. These were collected together after his death as the Pensées de M. Pascal sur la religion, et sur quelques autres sujets (Thoughts of Mr. Pascal on religion and on some other subjects). The Pensées, despite their fragmentary character, have become a world classic.
Because it consists only of notes we always have to be very careful in interpreting claims we find in the Pensées. For instance, there are indications throughout the text that parts of the work were intended to be presented in dialogue form, or something like it, and therefore not every sentence in the notes may represent Pascal's own view, as opposed to a view that he thought he needed to consider. A good example of how this can affect interpretation is the most famous argument in the Pensées, Pascal's Wager. The notes related to the Wager are often read as if they were all part of a single argument; but there are indications in them that the Wager argument in its final form would have been dialogical, and that different parts of the argument are actually responses to very specific concerns by an interlocutor. (I've given a rough account of the way I think the Wager argument should be read here.)
Nonetheless we can get at least a rough idea of what Pascal intended for the work. The two major themes of the work, which would probably also have formed the two major divisions of the book, are the wretchedness of human beings with God and Jesus as the way for human beings to be united to God. Judging from the notes we have, it is very likely that the latter would also have included a defense of the Jansenist miracles, like the Holy Thorn, from their Jesuit detractors.
III. Reasons of the Heart
Traditionally, the 'heart' was seen as the seat of personhood. For instance, the scholastics often took the 'heart' to mean the intellect and will. It isn't clear whether Pascal had this specifically in mind, but Pascal's 'heart' is also the source of understanding and loving in the human person. The passage from which our dictum comes is the following note:
Le coeur a ses raisons que la raison ne connaît point; on le sait en mille choses. Je dis que le coeur aime l’être universel naturellement, et soimême naturellement selon qu’il s’y adonne; et il se durcit contre l’un ou l’autre à son choix. Vous avez rejeté l’un et conservé l’autre: estce par raison que vous vous aimez? (#277)
The heart has its reasons, of which reason knows nothing; one knows it in a thousand things. I say that the heart loves universal being naturally, and also itself naturally, insofar as it gives itself to them; and it hardens itself against one or the other at its own choice. You have rejected the one and kept the other; it is by reason that you love yourself?
Note that the emphasis here is not on our dictum but on the two kinds of love: love for universal being and love for one's self. And the point is that the latter is not girded by reasoning, calculating out the reasons for loving oneself: one simply does it, naturally, and one can only fail to do it by deliberately hardening your heart against yourself. Likewise with love of universal being. The heart involves instinctual impulses, fundamental principles of life; they are not reasoned out, but they are also not irrational. We see this when we ask what sort of things Pascal thinks the heart can know. And, mathematician that he is, he is very clear: one of the things we know by heart rather than reason is mathematics:
We know truth not only by reason but also by the heart; it is in this last way that we know first principles, and reason, having no part in it, tries in vain to combat them. Pyrrhonians, who have nothing else for their object, labor uselessly. We know we do not dream; however powerless we may be to prove it by reason, this poweressness shows nothing other than the weakness of our reason, not the uncertainty of all our knowledge, as they claim. For knowledge of first principles, such as space, times, motions, numbers, are as sure as anything that our reasoning gives us. And this knowledge of the heart and of instinct are such that reason must trust it, and base all of its discourse on it. (The heart feels that there are three dimensions in space and that numbers are infinite and reason then shows that there are no two square numbers of which one is double the other. The principles are felt, the propositions are concluded, and all with certainty, although in different ways.) (#283)
If we had stronger and better minds, we would, in fact, know everything by heart, the way we know our first principles. But our minds are actually fairly weak. So some things we know by heart, such as the things without which we cannot even make sense of our own reasoning, and from these fundamental things reason draws its conclusions. If we did not have the heart to anchor our reasoning, all our reasoning would be purely hypothetical: it is the heart that, without having to reason, reasonably accepts some things as certainly true and some things as certinly false.
This ties into the theme of the Pensées in that Pascal argues throughout that God can only properly be known through the heart: it is by the heart, namely, through our love of universal being, that we know God, not by going through a step-by-step enumeration of reasons. This heart's knowledge of God is faith. Reasoning can have a role, in preparing the mind for faith; but the heart alone can sense God, and only when God draws it to love Himself. Pascal recognizes that there is a danger here: as he says elsewhere (#275), people often mistake their imagination for their heart -- they think they know and love when in fact all they are doing is imagining what it would be like to know and to love, just as, perhaps, some people confuse understanding mathematics with imagining that one understands it. But this danger does not show that there is no such thing as knowledge of the heart; rather, it shows the importance of finding true knowledge of the heart rather than merely imaginary knowledge.
This context perhaps gives a better idea of what Pascal means when he says the heart has its reasons whereof reason knows nothing; it's not about fuzzy sentiment but about fundamental understanding and love, the kind that makes reasoning possible in the first place. It is not based on reasoning; rather, it is that on which reasoning is based. And despite not being based on reasoning, it is also eminently reasonable, since reason gets its own reasonableness from it. The heart goes beyond reason because it is the very environment in which reason operates, without which it could not even exist. Some things are beyond our power to reason out; they must be simply understood.