Certain forms of materialism have been endorsed by many philosophers. According to some of these theories, each person is to be identified with his or her body. The body is the bearer of both physical and mental properties. The person persists so long as his or her body persists. Chisholm, however, was always uneasy about such theories. One of his most distinctive lines of objection involves the use of some metaphysical distinctions. Consider a ship that persists through successive replacements of parts. First one plank is replaced, then another, until after some time many of the ship’s original parts are no longer present. Chisholm describes the situation by saying that at each time that the ship exists, it exists in virtue of the fact that a certain collection of parts exists then. The collection of parts that exists at a moment is an “ens per se” — it exists in its own right; but the ship is an “ens per alio” — it exists in virtue of the fact that some other things exist. Different collections of parts do duty for, or stand in for, the ship at different times. In some cases, a thing has a property at a time in virtue of facts about happenings at other times. Thus, for example, a person might be a widow at a certain time, but that’s because of marriages and deaths that happened earlier. Other properties are not in this way "rooted" in facts about other times. With respect to properties that are not rooted outside the times they are had, Chisholm makes an interesting claim about a difference between things that exist in their own right and things that exist in virtue of others. The ship (an ens per alio) has properties at times in virtue of the fact that its stand ins (entia per se) have those properties at those times.