Essentially Marx is saying that looking at population would be a very generic image of the whole.
By looking at it like this empirically, concrete would dissolve into abstract. State --> DOL, Money, etc

HE DOES SAY LATER THAT THIS METHOD DID HAVE HISTORICAL VALUE
(THIS WAS THE METHOD OF 17TH C – PETTY AND DE BOISGUILLEBERT)

18TH C Economists
Marx says “there began the economic system, which ascended from simple relations such as labour, DOL, etc, to the level of the state, exchange between nations and the world market.”

So essentially, if you have the right categories, then Marx states “the journey would have to be retraced until I had finally arrived at the population again, but this time not hte chaotic conception of a whole, but a rich totality of many determinations and relations”

Musto kind of puts a sidenote here, and says that theres been a lot of different commentaries about this, but although Marx says that this is the “scientifically correct method”, this does not mean it is the one he emploeyd.

Musto says that Marx didn't believe as strongly as other economists that reconstructing concrete at the level of ideas was a real reproduction of reality.

MARX SHARES IDEAS WITH HEGEL, BUT THERE ARE RADICAL DIFFERENCES – not going to really discuss that.

Marx continues in the introduction by positing could simple categories exist before concrete ones?
NO – possession of propery, couldn't have existed before something more concrete like family.

But then Marx discusses stronger examples like money and labour... and he says about money – it existed before capital... but its use was primarily for trade – thus it shows it was historical.

Another simple category, and one that is a lot more complicated and Marx spent a lot of time discussing, is labour. He says, “Labour seems a quite simple category. The conception of labour in this general form – as labour as such – is also immeasurably old. Nevertheless, when it is economically conceived in this simplicity, ‘labour’ is as modern a category as are the relations which create this simple abstraction.”

Marx also draws this conclusion from labour because when it is economically conceieved, labbour is as modern a category as are the relations which create this simple abstraction. Thus, in societies today, labour transfers with people, there is no character to it or specific craft... and in regards to wage labour it is purely abstract labour.
And i mean, this is pretty clear today,

Today, when we say labour, it doesn't refer to one job or work, theres so much workers and so much jobs. Lawyer, factory worker, construction worker, etc.

Skillwise – they're pretty unrelated, so now this is kind of an abstract labour. We either think of these jobs as a totality, or individually under this heading of 'labour” - the categories have different meanings in different societies

a really good quote here by marx again is when he says “This example of labour shows strikingly how even the most abstract categories, despite their validity – precisely because of their abstractness – for all epochs, are nevertheless, in the specific character of this abstraction, themselves likewise a product of historic relations, and possess their full validity only for and within these relations.”

Now, Marx turns to the final part of this section. How do we set out these categories? In what order? He says we can't do it historically, we can't order them historically, rather, we have to order them by how they are ordered in modern bourgeois society, which he says “is precisely the opposite of that which seems to be their natural order”... and he concludes with

1) the general, abstract determinants which obtain