David Hume did not assume full employment.

Inflation and Disinflation in Turkey, edited by Faruk Selcuk, Libby Rittenberg, Aykut Kibritcioglu:

O'Brien (1975) argues that there are some differences between transmission mechanisms in classical and neoclassical versions of QTM. The neoclassical model is based on the assumption of full employment, and is characterized by a dichotomy between the real and monetary sectors. Real wages will be determined in the real sector (labor market) while nominal prices are a function of the money supply. Therefore, increases in the money supply increase the general price level by leaving the volumes of goods demanded and supplied, and hence, real output unchanged. 

On the other hand, O'Brien writes, some classical economists like David Hume do not assume full employment and there is no room for a dichotomy. According to Hume, an increase in the money supply does not increase the general price level through a different transmission mechanism. The increase in nominal cash balances of economic units initially results in higher expenditures for goods, and hence, in higher production. Then, under the assumption of underemployment, prices start to adjust to risen money supply. As a result, money is not neutral as in the neoclassical model; it has also some real effects in the short run.

 
David Hume did not assume full employment. 

Milton Friedman did. Friedman in The Counter-Revolution in Monetary Theory, page 8:

If the government gets the funds by borrowing from the public, then those people who lend the funds to the government have less to spend or to lend to others.

Those who lend to government have less to lend to others.

Q: At what time in history (since Charlemagne, say) would lending to others have reached its "full employment" stage?

A: It would have to be the time that Keynes referred to in Chapter 23 as "the greatest age of the inducement to investment" and in Chapter 21 as "a period of almost one hundred and fifty years" when "rates of interest were modest enough to encourage a rate of investment " that was "consistent with an average of employment which was not intolerably low."

"[N]othing short of the exuberance of the greatest age of the inducement to investment could have made it possible to lose sight of the theoretical possibility of its insufficiency."

-- J.M. Keynes

At such a time, it would have seemed that "full employment" had been achieved.

When, exactly? I'd say from the publication of The Wealth of Nations to the First World War: the 138 years from 1776 to 1914.

During that time, it seemed reasonable and natural to think full employment had been achieved. Investment was viewed with an optimism that now seems unnatural. It seemed self-evident that "supply creates its own demand". And economists lost sight of the theoretical possibility that investment could be insufficient.

However, economists this side of the first World War have no such ready justification for assuming full employment.

If you think of civilization as a massive business cycle some 2000 years in length, the 150 years of the "greatest age" make a nice high point. This coincidentally puts Charlemagne just at the bottom, where he was busy starting the upswing of the cycle.

This is why the economy changes: It is part of a massive cyclical phenomenon. Economic forces change and sometimes fade; and other driving forces (religious, political,  military, and irrational) may also each have a dominant phase. But the cycle is a handy framework for thinking about the economy.

And if you think of economic forces as among the forces that drive the cycle, you can see why David Hume (1711-1776) did not assume full employment, and perhaps why J.B. Say, just a little later, did.

It also becomes obvious that Milton Friedman shouldn't have. Nor should we.