Logic is a tricky thing to evaluate. One of the earliest approaches was laid down by Aristotle, in the form of the syllogism. All are statements of relationships and, if the premises are true, there are only certain conclusions that may logically follow. Every chemist is a man. Therefore, every chemist is a lover.
Every m is a, Every b is m.
Therefore, every b is a and it is a valid mood called Barbara. Of the 256 possible combinations of all's and some's and none's and are's and are-not's, only 24 moods are valid. The conclusion is the same. But the second premise is false so the entire reasoning is illogical. It is not a fact that every stimulant is a phentermine.