At the risk of flattering Isaiah Berlin, who I believe to be a very overrated figure, it is important in debates about liberty to not muddle two different concepts.
Liberty in the “negative” sense refers to essentially non-interference. Previously, I have spent a lot of time pointing out that this sense of liberty is incompatible with property because property involves interfering with those deemed to be non-owners. Under normal property theories, when pieces of the world are unowned, they may be accessed by anybody without interference. When those pieces of the world are made owned by appropriation, this goes away and a massive scheme of interference is put in place to interfere with anyone who tries to access the appropriated piece of the world. Whereas previously everyone was at liberty to access and use that piece of the world (provided they don’t physically interfere with the body of someone else), they now no longer are. Under “negative” liberty, then, property is the most anti-libertarian institution in history.
Liberty in the “positive” sense is a more muddled concept. But one way to think about it is as indicating the full set of things you can actually do. A libertarian not happy with the conclusion of the prior paragraph may note that, although property does restrict you in a purely physical sense, the institution of property facilitates the creation of so much wealth that it really does give you more genuine liberty to do stuff. Put another way, in the propertyless world with totally unrestricted access to everything, the value of the stuff you can access may only be worth $500. But, in the property world, the value of the stuff you can access may be even on the low end $10,000. So though property might look initially like a great infringement of liberty (defined conventionally), it actually is a great expander of liberty (defined in basically welfare terms related to the index of things a property system eventually allows people to do).
I am not going to quibble here and talk about real liberty, but it’s important to tease out the implications of the “positive” liberty move. What it means is that liberty is not a binary thing related to whether you are being restricted by others or not. Instead, liberty is a sliding scale concept that, crucially, is related directly to how much money (or whatever you want to use as the stand in for resources/stuff) you have. This means poor people have the least liberty and rich people have the most liberty. This means that transfer incomes increase the liberty of their recipients and, in that sense at least, are distinctly pro-libertarian measures. This means that “equal liberty” (something libertarians generally seem to be supportive of) requires economic egalitarianism.
While I don’t want to problematize the second notion of “liberty” per se (in fact I think it more closely tracks on to what is meaningful about liberty), it’s worth noting that it’s basically defined liberty as welfare. Under the first definition, we can imagine people with significantly different ability to do things and access things in the world but who have equal liberty insofar as they are not subject to external restraint. Under the second definition, this is not possible. In this sense, it becomes a lot harder to understand what the “libertarian” answer to certain things is. Cutting food stamps reduces the liberty of food stamp recipients, but may increase the liberty of whoever’s pocket the money winds up in instead. How do we know what the libertarian approach is? Is libertarianism about maximizing liberty so defined? Does liberty maximization contain within it an appreciation of diminishing marginal liberty (utility)?
Shifting to a positive sense of liberty might save you from the anti-propertarianism inherent in the negative sense of liberty, but trying to then use this positive sense of liberty to get you to laissez-faire capitalism (the libertarian’s ultimate goal) is fraught with problems.