There is a well known thought by the French 19th century thinker Proudhon that "Property is theft," and so to a quick look at this in a linguistic sense to see how much sense it makes. If someone makes a mathematical claim it has to of course be consistent with mathematical language, and similarly statements in this 'ordinary' language should be treated much more seriously as being exactly what they existentially are, ie language statements. Words possess meaning, and so what is the meaning of the relevant words?
"Property is theft." Though it might appear in jest I think this could and should be accurately seen as a very childish semantic error. Property and theft are two different words meaning two very different things. If Proudhon had said "Property is property," then, though this inane pronouncement would have surely had no future fame, it wouldn't have abandoned sense. And similarly if he had said that "Theft is theft." However Proudhon ventured into the more strange waters of declaring one word to in perfect truth imply a very different word, i.e. that 'theft' and 'property' are one.
Though as someone is sure to helpfully clarify that what Proudhon is trying to say is that property is itself immoral because of being the equivalent to the act of theft; that somebody owning something is by definition stealing from someone else.
However, stealing from someone can only exist where property exists in the first place, and so the idea that 'property is theft' is entirely unintelligible. To use the word theft is to necessarily accept the legitimacy of property - theft being the illegitimate taking of someone else's property. And so to try to say that property is theft is self-contradictory gibberish.