There are many branches of liberal thought, and much disagreement on how to classify the different liberal perspectives. One way to distinguish between them is by the importance they attach to private property.
"Classical" liberal thinkers view private property rights as massively important. It just so happens, incidentally, that by protecting private property rights wealth is maximised and everybody tends to be better off. It also incidentally happens that individual liberty is maximised, and people live lives that are more free. But these outcomes are purely incidental - private property rights are worth protecting for their own sake. This means that it wouldn't be ok to abolish private property rights because you think you've discovered a different method of maximising wealth or because the majority of voters have decided that they don't care about liberty.
To "progressive" thinkers, fair distribution of wealth is much more important than protecting private property rights. It just so happens, incidentally, that some measure of private property is perfectly fine and acceptable and produces pleasing results like wealth so private property rights should be tolerated to some extent (no crazy communism here). But in this school of thought the beneficial effects of private property are merely incidental and quite unimportant. The more important goal is to ensure that wealth distribution is as equal as possible, so that it's fair. We all want to live in a fair society, right? Also, having fairness as your goal sounds vaguely more moral and upstanding than pursuing liberty.
Thus the disagreement is not whether private property rights should exist, or whether they should be protected. The difference lies in the priority that should be given to protecting private property rights:
A 4-stage hierarchy of private property protection:
1. Absolute priority: protect private property at all costs, no exceptions. Wealth distribution is absolutely irrelevant. Justification: individual economic liberty. Government has an extremely limited role: to protect life, liberty and property. No need for taxes, and actually if citizens are armed like in some US states you might not need a government at all.
2. High priority: protect private property at all costs subject to limited and specified exceptions e.g. if someone's life or liberty is at risk. Wealth distribution is mostly irrelevant, but sometimes taxes are needed to help out the poor e.g. to provide free schools. Justification: individual economic liberty. Government has a limited role: to protect life, liberty and property.
3. Moderate priority: protect private property alongside other claims and expectations; all claims have equal priority. Equal distribution of wealth is desirable, and just as important as private property protection. Justification: social cohesion and social justice. People get along better when they're not consumed by envy and jealousy: you can achieve this by ensuring that nobody is allowed to become richer than the others. The role of government is much bigger: alongside protecting basic rights there is a role in wealth redistribution. You also need bigger government so they have enough officers to monitor everybody's pay and make sure nobody is earning too much money.
4. Low priority: protect private property only in situations where nobody feels that it's unfair. Equal distribution of wealth has high priority. Justification: wealth egalitarianism and social justice. Government has a massive role: to redistribute everything and show us how to get along and play nicely together, sharing all our toys.
In this hierarchy a classical liberal would assign absolute or high priority to property rights, while a progressive liberal would assign only moderate or low priority.
Why do classical liberal thinkers consider property rights to have high or absolute priority? There are two possibilities:
First possibility: simply because they're a bunch of right greedy hogs. Being greedy is very very bad, so if this explanation is true there's not much more to say except to damn their mortal souls.
Second possibility: because of the importance they attach to liberty, and the importance of private property in safeguarding liberty. If liberty is the reason, the debate becomes a bit more interesting.
The importance of liberty
If you were a slave, or a feudal serf, or an indentured labourer, I guess nobody would need to convince you about the importance of liberty.
But it's the twenty first century, and these days we're all free so liberty doesn't seem all that important. If you live in a welfare state it's tempting to suppose that getting free money from the government through wealth redistribution is worth more in your daily life than liberty. After all, you can't use 'liberty' to buy nice stuff in the shops, so a bit of free money would be much more useful. If you don't live in a welfare state then you might think liberty is for the birds, and you'd much rather make your way to a rich country where the government redistributes wealth and looks after everyone. Now liberty is not the goal. Equality is much more important.
Classical liberals take a different view: liberty is what makes your life worth living, helps you discover your personal talents, and build a life you can be proud of.
This explains why classical liberals value liberty above redistribution. Their perspective differs so completely from that of 'progressive' liberals that it seems scarcely correct to define progressivism as a strand of liberal thinking.
Scholar, Writer, Friend