A couple of days ago, Dr. Eugenia Cheng, @DrEugeniaCheng, who is always worth reading, tweeted something that really made me stop and look again:

Omg I occasionally click on the “recommended” articles on my firefox homepage despite my best intentions, and they are universally terrible. I have finally got up the ertia to work out how to stop them appearing.

Ertia! I was, needless to say, plussed. Gruntled, in fact, and entirely combobulated, though a bit chalant. Such a sensical and ept word – and quite feckful and ruly, too. You would expect it to be more inlandish. But it is just not in regular use.

To be fair, inertia doesn’t come to English as the negative of ertia. We borrowed inertia straight from Latin, in which it is the noun form of the adjective iners (inert, on the other hand, comes from French inerte, which came from Latin iners). So does that mean that the if you’re not iners you’re outers? No, and not exers, either. The in is from Latin meaning ‘not’ (we see it in words such as insufferable, and often get confused by the intensifier in as in inflammable). The ers is a modified form of ars, which is the source of our word art – but in Latin it meant ‘skill’ or ‘strength’ or ‘craft’.

So iners meant ‘lacking in skill, craft, or strength’ – I think the closest English word in meaning might actually be feckless – and inertia thus referred in Latin to a kind of lassitude, lumpishness, or indolence. The sort of state in which you need someone to light a fire under your ars. But it arrived in English thanks to Isaac Newton in 1687 in the second definition at the very start of his Principia Mathematica:

Materiæ vis insita est potentia resistendi, qua corpus unumquoque, quantum in se est, perseverat in statu suo vel quiescendi vel movendi uniformiter in directum.
Hæc semper proportionalis est suo corpori, neque differt quicquam ab inertia massæ, nisi in modo concipiendi. Per inertiam materiæ fit, ut corpus omne de statu suo vel quiescendi vel movendi difficulter deturbetur. Unde etiam vis insita nomine significantissimo vis Inertiæ dici possit.

Here’s Andrew Motte’s translation of that:

The vis insita, or innate force of matter, is a power of resisting, by which every body, as much as in it lies, endeavours to persevere in its present state, whether it be of rest, or of moving uniformly forward in a right line.
This force is ever proportional to the body whose force it is; and differs nothing from the inactivity of the mass, but in our manner of conceiving it. A body, from the inactivity of matter, is not without difficulty put out of its state of rest or motion. Upon which account, this vis insita, may, by a most significant name, be called vis inertiæ, or force of inactivity.

In other words, matter is just plain lazy. It has no intrinsic motivation to change its state of motion.

Fortunately, there’s always art – and craft. And art, it seems, is self-motivating: Ars gratia artis (‘Art for art’s sake’), as Metro-Goldwyn-Mayer’s motto goes. So just as the inertia of the fixed form of the word is acted upon by the craftiness of Dr. Eugenia Cheng (and others) to make ertia from inertia, our artfulness and craftiness change the state of matter throughout life.

Oh, but be careful. Ars longa, vita brevis, you know. (And no, that doesn’t mean ‘briefs are vital when you have a long arse’.) That has often been taken to mean that our art will long outlast us – giving it a staying power ironically suggestive of inertia – but really it means that it takes a long time to learn a craft, but you only have so many years before you’re inert. Here’s the Latin source:

Vita brevis,
ars longa,
occasio præceps,
experimentum periculosum,
iudicium difficile.

And here’s the original from Hippocrates, which was translated into the above Latin:

Ὁ βίος βραχύς,
ἡ δὲ τέχνη μακρή,
ὁ δὲ καιρὸς ὀξύς,
ἡ δὲ πεῖρα σφαλερή,
ἡ δὲ κρίσις χαλεπή.

Which means, in the uncredited translation on Wikipedia:

Life is short,
and art long,
opportunity fleeting,
experimentations perilous,
and judgment difficult.

But that ars is Latin ars, and it translates Greek τέχνη tekhné, which referred to craft and technique – and is the source of technique and technology.

Sorry, folks, once I start digging, it’s hard to stop. But what, then, is ertia? It’s a display of technique, craft, skill. And you could say it means ‘technique, craft, skill’ too. Or at least ‘get-up-and-go, stick-to-it-iveness, and get-there-ativeness’. Which we could all use more of.

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