Slavery, albeit absolutely wrong, wasn't 'illegal' since the Constitution, being the Supreme Law of the USA
Incorrect, the Constitution is not and was not the Supreme Law of the USA. It never has been, and it never will be. In fact the drafters of the US Constitution took great pains to point that out when they wrote up the Declaration of Independence. In that document they espoused that universal human rights are superior to ANY constitutional arrangement, and used the supremacy of human rights as the justification for overthrowing the government, and in so doing, denying that their actions were treasonous.
Virtually the same theory was used to prosecute the Nazis at the end of WW2. They had drafted up a constitution for Germany that made the holocaust legal. Despite that, everyone recognized that a nations constitution is in fact NOT the supreme law, that there are universal human rights that trump any constitutional provision. On that theory the Nazis were tried for crimes against humanity, convicted, and jailed.
So not only have the authors of the US Constitution gone to great lengths to explain that constitutions are in fact not supreme law, but there are clear examples in history of leaders of nations being convicted and jailed for crimes against humanity where their national constitutions included no such provisions for any such crimes.
You can go back to the Treaty of Paris in 1815 as well when the topic of slavery, our topic, was debated in the British House of Commons and it was concluded, and ratified, that it is illegal because it represents a violation of human rights that is so odious that it is an affront to human dignity.
So yes it was illegal, and no it doesn't matter WHAT the US Constitution had to say on the topic, because it's well established, including by the drafters of the constitution, that constitutions of nations simply do not have the power to revoke universal human rights.
He doesn't need an argument - he's Thomas Jefferson, one of the great founding fathers and intellectuals in American history.
He's a criminal who should have been jailed for horrific crimes against humanity, and really, we should all go spit on his grave. Anyone who treats another human being the way he did is not worth a second thought. As a degenerate slave owner the man has not got an opinion on freedom that is worth listening to.
Can you prove Jefferson's statement wrong?
I can think of many reasons why it's wrong. Once we get past the point that Jefferson's association with the idea is a strike against it we can go on to talk about why in fact it is often necessary for governments to raise taxes.
We can start by discussing what are the appropriate metrics by which to judge such things: I will put forward economic efficiency with some limits that respect those universal human rights. So for example if it turns out to be more efficient to implement a welfare system then that is what we should do. Now let's leave aside for a moment the question of whether welfare is really more efficient or not--I am using it as an example mostly to illustrate the point that there ought to be no sacred cows.
Whether it is more efficient in fact, or isn't more efficient in fact, is something that we should measure empirically and scientifically and based on solid data we should choose the policy that maximizes efficiency provided that it does not infringe greatly on a fundamental freedom. Thus I leave aside for now the question of WHAT policies we ought to implement, and simply propose the method by which we ought to select such policies.
Specifically, we should never accept or reject a policy for no better reason than some ideology is for or against it--ideologies have the status of religions in my eyes, fanciful fairy tales told to children to help them sleep at night, but not worth considering in serious political discussions.
I thought you implied it. Let me try to understand you. So you question Jefferson's 'role of government' quote on the basis of his civil rights violations, but accept his DOI and preamble on human rights?
I find it quite effective to use his own words against him.
Isn't it simply because you disagree with his conservative economic policies or theories?
Nope. I honestly don't know what Jefferson's economic policies or theories are, other than this quote, so that can't be it.