I'm reading from Dirk Van Dalen's Logic and Structure and in that the following theorem occurs
Let $A$ be a property, then $A(\varphi)$ hold for all $\varphi \in PROP$ if
- $A(p_i)$, for all i and $A(\bot)$
- $A(\varphi)$, $A(\psi)$ $\implies$ $A( ( \varphi \square \psi) )$
- $A(\varphi) \implies A ( \neg (\varphi) )$
Now, I have few doubts regarding the way things are written:
- Why is he writing just "holds" not "holds true"? For example: "then $A(\varphi)$ holds", it should be written "then $A(\varphi)$ hold true"
- Why the same symbol $\varphi$ in the actual statement and then in point 2 and 3? I think he should use different symbols.
Now, the proof of it goes like this:
Let $X = \{ \varphi \in PROP | A(\varphi) \}$, then $X$ satisfies (i), (ii), (iii) of definiton 1.1.2. So, $PROP \subseteq X$, i.e. for all $\varphi \in PROP$ $A(\varphi)$ holds.
Can someone please explain the proof to me? I really have no idea, first he assumes $X$ to be the set of those propositions which are in PROP and for whom $A(\varphi)$ is true, well this clearly shows that $X$ will be a subset of $PROP$ but just in the next line he shows that $PROP$ is a subset of $X$. I'm quite confused.
DEFENSE: Other questions do exist for the page and theorem of book but their doubts are different from mine.