As it happens, The Whole Equation is itself a deeply lovable book - passionate, thoughtful, unpretentious, even a little dishevelled in its excitement at a medium which Thomson argues is at its best when its motives are mixed. It is a reminder, too, in an age when spectacle often comes first, that movies are never better than when they get behind our eyeballs and begin to work on our private life.
I couldn't help calling it to mind when I saw a trailer for the latest of Channel 4's essays in endurance television - this weekend's 100 Greatest Tearjerkers, which will occupy an eye-watering four hours of the Sunday evening schedule. As is usual with these things, the implication that the chart is the result of a popular plebiscite is spurious. Viewers could vote on their favourite moments, but only from a list pre-selected by the channel, so it might more accurately be titled 100 Greatest Tearjerkers for which Channel 4 Has Managed to Secure Screening Rights. And as the nominees include Terminator 2, Escape to Victory and Pop Idol, it's hard to believe that emotional subtlety or even cinematic excellence has much to do with it.
But, whatever its validity as a poll of snuffly opinion, the programme should provoke a variation of Thomson's question about love. When we cry in the cinema, what exactly are we feeling - and how trustworthy is the sentiment?
There is a kind of oddity here. You might crudely take weeping in the cinema as a kind of affective antonym of laughing in one - and yet there's absolutely no symmetry in the way that we feel about these different bodily reactions. While it's not at all uncommon for people to feel a vague sense of resentment at being made to cry by a film and to resist before succumbing to scenes which manage the trick, I doubt that any movie-goer in the long history of cinema has ever said: "I roared with laughter... but I felt emotionally manipulated afterwards." Even the slang we use to describe such films conveys the wariness we feel; to have your ribs tickled is a lot less unsettling than having tears jerked from you. One feels like a gift, the other like a kind of theft - something extracted without your full consent.
This is partly down to the simple fact that it feels a lot better to laugh than it does to cry - however dolorously pleasurable we find our fictional griefs. But I suspect, too, that it has something to do with our emotional prehistory - since the consequences of being fooled by false displays of grief and pain are far greater than the consequences of bursting out laughing when a fellow cave-dweller trips over a mammoth pelvis.
It matters rather a lot that we can trust our susceptability to pity and empathy - so when these emotions are stimulated in conditions that we know to be highly artificial we're left uncertain. And while laughter can often feel as if it has bypassed the conscious mind, like some physical reflex, tears are almost always contemplative. There is actually such a thing as a belly-sob - a sudden, unforeseen jolt of sorrow - but it's far, far rarer than its comic equivalent.
Perhaps most important of all, we feel that tears reveal something about us in a way that laughter doesn't. And that's because tears in the cinema are always personal. Laughter connects you to the people sitting next to you, roaring away without inhibition. Tears connect you to the person sitting inside you, furtively trying to blink away the evidence of your emotional capitulation. What's more, whereas laughter is always in the present tense, you know at some level that your weeping is a rehearsal - either a testing of griefs you haven't yet felt but know might be on their way, or a welling-up of sorrow that you thought had been safely absorbed.
That's when you actually let the film take its time, of course - not when you dismember it for the purposes of schedule-padding. On Sunday night I doubt there'll be a wet eye in the house.