In his article on whether almsgiving is a matter of precept (a precept is a law that binds under penalty of mortal sin, so following it is necessary for salvation, while a counsel is a means to a perfection greater than what is necessary for salvation), St. Thomas begins his response to the objection that, since "it is lawful for everyone to use and to keep what is his own... it is lawful not to give alms," with this:
The temporal goods which God grants us are ours as to the ownership, but as to the use of them, they belong not to us alone but also to such others as we are able to succor out of what we have over and above our needs.
So we have ownership of our goods, but not free use of them.
Isn't this just the sort of distinction that drives some people up the wall? What's the point of my saying you own something if you can't stop me from using it? It sort of sounds like one of those stories of a burglar who sues his victim because he stubbed his toe on the victim's coffee table.
For St. Thomas, though, the distinction between owning something and using it was far from academic. As a member of a religious order, he himself was forbidden to own anything. In fact, he goes so far as to write "that in the attainment of the perfection of charity the first foundation is voluntary poverty, whereby a man lives without property of his own"; in other words, the evangelical counsel of poverty is a primary and necessary part of religious life. But a religious can still use things he doesn't own: the bed he sleeps in, for example, or the prayerbook he uses in the chapel.
I won't say it's unthinkable that a religious might come to regard something he uses as something he owns. Nor is it unthinkable that a secular person might come to regard something he owns as something that is necessarily his alone to use. But if we are to understand what St. Thomas teaches about temporal goods, we need to understand that the one doesn't always imply the other.
So what does ownership always imply, if not free use? "Stewardship" may be a better way of thinking about possessing temporal goods than the contemporary notion of "ownership," which would be expressed in terms of individual rights. What I own is for me, not anyone else, to dispose of, but that doesn't mean it's for me to dispose of however I want. I am, you might say, the rightful authority for determining what's to be done with what I own, but mine is not an absolute nor sovereign authority. In the case of my surplus or your need, I am bound by precept to let you use my goods (which may involve transferring ownership to you); if I fail, then under certain circumstances my ownership is forfeit and the goods are yours to take.