is just saying $p$ equivalent to saying $p$ is true? In my textbook modus ponens is explained amongst Rules of Inference. It is written as:

$p\rightarrow q\\ p\\∴q$

But $p$ is just a proposition— by definition, it is a statement that is either true or false, but not both. So how can you say that the statement $p$ is true? Does just making some proposition on its own line mean the proposition is true?
Edit
Does just making some proposition on its own line mean "if the proposition is true"?
 A: Modus Ponens, as well as other valid argument forms, guarantees the truthfulness of its conclusion (under the assumption that the premises are true), but it does not guarantee the truthfulness of its premises. This will become more clear if we review the definition of an argument form and validity.
Modus Ponens is a valid argument form, meaning it is an arrangement of variables and operators such that the uniform replacement of the variables by statements results in an argument that is valid. Recall the definition of valid: an argument is valid if and only if it is not possible for the premises of an argument to be true and the conclusion to be false; this implies that if the premises are true, then the conclusion must also be true. 
Thus, Modus Ponens makes no claim about the truthfulness of its premises: $p \rightarrow q$ and $p$. All it means is that "if both premises $p \rightarrow q$ and $p$ are true, then the conclusion $q$ must also be true." In otherwords, $q$ is definitely true if the premises $p \rightarrow q$ and $p$ are true. If at least one premise is false, then no such guarantee can be made.
