Property is not discovered; it is made, not by the act of mixing labor with supposedly ‘fallow land,’ as Locke would have had it, but by the scaffolding provided by the surrounding legal system.
So an agreement, designed according to what makes sense. Of course, this "sense" gets rather complicated.
The simple story is that property should be fair, distributed according to one's desert. However, how to establish desert? If I inherit a million dollars, I certainly did nothing to deserve it, in that I played no part in its creation. But maybe it is fair to respect the wishes of the deceased. But what if they inherited it, and so on?
Let me toss another piece of wood into the fire: as a behaviorist, I can make a rather solid case that all of our actions in life are in a sense "inherited", in that they are entirely a function of our genes and our environment. As such, any action we take to create wealth is inherited.
This may seem a fanciful stretch, even if you accept the premise that our actions are not our own. Surely we must act as if they are. As a practical matter, maybe this is true. However, we certainly don't act this way at a societal level. In criminal justice, people are judged to be responsible for their actions, and thus deserving of a range of punitive measures. In our economic system, people are assumed to have "earned" their fortunes - or lack thereof. As such, property is hardly given a second thought as the direct result of personal action.
If our actions are inherited, then all forms of property inequality (not to mention other forms of capital) are injust. As a practical matter, remedying this injustice in a complex society is obviously no easy task. History is riddled with horrific results of experimentations in equality. However, it is also filled with examples of successes (public schools, libraries, parks, social security, medicaid, etc.).
Many of our political arguments are over the practical effects of social responses to equality - whether or not they would work, whether we can afford them, whether they have secondary negative effects and so on. Yet, first we must establish whether or not there is a moral imperative, a problem to address. And at this point, a good-as majority of the country simply disagrees with the premise that we inherit our actions, much less that social interventions might be effective.
(A strange irony is that many of these very same people view social interventions as having negative effects on motivation, which is a completely behavioral analysis, and would as such seem to agree with the premise they deny in the first place!)