There are many different philosophical ways to arrive at an economically leftist political position. One of those philosophical approaches — which I think has been somewhat neglected — is centered on the issue of property ownership. Unfortunately, many — even on the left — will concede that property rights exist, and that the institution of property makes sense. Those on the left who accept property rights typically argue that those rights are qualified by some other countervailing social concerns.
I think this is the wrong move: the issue of property should be attacked head on for the incoherent mess that it is. In the above video, G.A. Cohen gives a very simple explanation of the issues with property ownership. Political conservatives — especially libertarians — really like to emphasize the right of individuals to own enormous sums of resources by appealing to certain processes. They will typically talk about voluntary transactions and mutually beneficial exchange.
These talking points have all sorts of responses, but the quickest one is just to attack ownership outright. You cannot justify ownership based on free exchange because ownership necessarily does not originate from free exchange: at some initial point, someone had to just grab some piece of land without exchanging with anybody. This is logically unavoidable.
So the question then becomes: how can that possibly happen? If property is justified by voluntary exchange, then how can property ever come into existence at all given that the first owner did not voluntarily exchange with anybody? Now, there are all sorts of efforts to explain how that initial appropriation can occur. Philosophers like John Locke, Murray Rothbard, and Robert Nozick give famous accounts, and there is significant amounts of literature explaining just how spectacularly they all fail.
But the easiest way to understand how original appropriation cannot be justified within a conservative/libertarian framework is by focusing on the idea of opportunity loss. When an individual declares perpetual ownership of some piece of unowned land, every other human being on earth suffers an opportunity loss: their opportunity to use that land has now disappeared. Opportunity losses are real economic harms.
To be concrete about this, consider an example. The piece of land down by the river is owned by no one; so everyone can use it. Sarah declares — on whatever property theory she prefers — that the piece of land by the river now belongs to her exclusively. But, wait a minute. The previous ability of others to use the land by the river has now vanished! They have been hit with opportunity losses. If one of the dispossessed were to say “this is silly, I do not consent to giving up my pre-existing opportunity to use the land down by the river,” Sarah uses violence (typically state violence) to keep the dispossessed out.
Unless unanimous consent exists, the original grabbing up of property results in violent, non-consensual theft from others. It is really just that simple. What follows from that conclusion is that the conservative/libertarian positions that depend on the sanctity of property rights are totally bogus. For instance, you cannot complain that taxes violently take material resources from you without your consent when property itself is predicated on just that. You cannot claim your enormous wealth was gotten fairly when the ownership of that wealth is predicated upon the non-consensual violence just discussed.
Given that this absolutist property rights position is totally untenable, the only remaining issue is what do you put in its place. Exasperated libertarians — unable to actually defend ownership — will typically shoot back that there is no other way to do it. If ownership is by definition a form of opportunity theft, then how do we ever move forward?
The alternative is not that complicated: subject resource use — both production and distribution — to social negotiation, i.e. democracy. This is basically the position of anarchists on property use. It is also at the core of left-liberal contractarian theories like those of John Rawls, and neatly folds into discursive democratic theories like those of Jürgen Habermas. The basic point though is simple: resource use and access is not something for which there is an objective answer; the answer is democratic decision-making. Now, we can roughly imagine what a democratic decision-making process about resource use would come up with. It would almost certainly be more egalitarian both in production and distribution than the system we currently have.
By rejecting the underlying property assumptions, we can open up a very clear avenue towards understanding that resource use must be governed by social negotiation. As I said at the top, this is not the only way to arrive at an economically left position, but I do think it is a particularly fruitful one.