Are we talking purely mathematically, in which case it is only true by definition, and it is still your opinion in how you are understanding that definition. For example in computing (not pure math) we have a saying "2+2 = 5 for sufficiently large values of 2" which is a play on some of the particulars of computing.
But something being true by definition is only true due to circular logic, and therefore true but not int he sense of being a world fact (a true idea).
By contrast though if you mean the real world manifistation of that.. so "if i have one rock, and pick up another, then I have two rocks" we still wind up in the same problem of true by definition but its just a harder concept to understand. In this case what is a "rock" and how we count rocks are ultimately at play and it still becomes a situation where it is only true by virtue of the fact that we define it as such. Again we would be in the problem that the definition still has room for definition... if i pick up a pile of sand, how many rocks do I have? One could argue each grain of sand counts as a rock.