There's far too much stuff in the world. Check Wikipedia if you don't believe me. Hit the "random article" widget on the left three times in a row, then guff your own legs off with amazement as it coughs up a trio of things you've never ever heard of before.

I've just tried this myself, and it introduced me to 1) an Australian Aboriginal tribe called the "Gunwinggu", 2) "Something Wicked", a 1993 album by Nuclear Assault, and 3) former computer games designer Demis Hassabis. Infuriatingly, I've actually met Demis Hassabis, so that's another theory left bleeding by the roadside.

Anyhow, the point I'm failing to make is that there's so much stuff out there, you can't possibly remember it all at once. Which isn't to say you delete it. Most of it you simply file away, somewhere at the back of your mind, by the bins. And there it stays for weeks, months, years, until something jolts you into retrieving it, at which point: bam! Instant recall.

I fear I'm not making sense, so let me explain. While watching the new David Attenborough series, Life In Cold Blood (Mon, 9pm, BBC1), I suddenly realised I'd completely forgotten about tortoises. I can't have thought about a tortoise in any shape or form for at least a year. They've been dead to me. Obviously, stuff like this happens all the time inside the human brain, usually zipping past unnoticed, but since I'm currently training myself to pay attention to every thought I experience, like a ninja, I caught myself being surprised by the realisation. I actually thought, "oh yeah, tortoises, I remember them".

But the beauty of a programme like Life In Cold Blood is that having reintroduced me to the concept of tortoises, it then astonishes me by depicting them doing something I've never seen them do before, namely fighting. Yes, tortoises fight. Did you know that? I bloody didn't. They've even got little jousting staves built into their shells, all the better for flipping one another on to their backs with.

Even the things you think you've seen before have a new and exciting twist. We see a python dislocating its own jaw in order to swallow a small deer. It starts with the head and slowly engulfs the whole body until it's coating the thing like a living condom. Familiar territory? Maybe so. But then Attenborough points out that the deer's head is so huge the snake can't breathe, so it sicks up its own windpipe and pokes it out the side of its mouth, like a floppy pink snorkel, puckering for air. Truly revolting. And new. And clever: it's the sort of thing Jack Bauer would do, if he had several million years to evolve his way out of a crisis.

Attenborough is routinely praised to the point where future historians might mistake him for a minor god, and quite right too. Few TV shows in any genre make you feel anything whatsoever, apart from a vague awareness that you're wasting your life, whereas his programmes, with their signature blend of understated commentary and magnificent footage, induce awe every five minutes. And not a sentimental, dewy-eyed kind of awe either, but a sobering one. In their own way, these are among the most nihilistic programmes on television. If your mind wanders at all during the tortoise fight, for example, it's likely to contemplate war, or terrorism, specifically dwelling on the extent to which conflict is an inbuilt human trait, just as it's an inbuilt trait in tortoises, which you'd previously thought of as a race of comically benign Cornish pasties, good for sleeping in boxes or appearing in the One Foot In The Grave title sequence, and not much else.

This is likely to be Attenborough's last major series: the final chapter in an extraordinary legacy. To change the way millions of people see the world is no mean feat, and he's done it with quiet assurance, humour, and respect.

TV can be many things. Nowt wrong with a bit of mindless entertainment now and then. But when someone with purpose seizes and commands it, it can also do this. Incredible.