Granting premises does not mean that every proposed premise has been established as fact. Logic can first assess the form of the inference and whether the conclusion follows.
Example:
All trees are made of iron.
All date palms are trees.
Therefore all date palms are made of iron.
The inferential form is valid: the conclusion follows if the premises are granted. But the first premise is false, so the argument does not establish a true fact about the world. Distinguish valid form from true premises.