A rough translation of the below picture.
Shaykh uthameen mentions a principle in fiqh
كل حلال فهو طاهر, وليس كل طاهر حلالاً, وكل نجس فهو حرام، و ليس كل حرام نجس
1) All halal things are pure, 2)but not all pure things are halal.
3) All impure things are haram, 4) but not all haram things are impure
1) All halal things are pure is clear.
2) But not all pure things are halal
Like harmful substances such as poison,alcohol (even though there is a difference of opinion on alcohol).
It is pure in itself but is haram to consume.
*pure meaning if it was on your clothes one could still pray.
3) All haram things are impure
Logically it can be understood: If something is impure and must be removed from our bodies and clothes, then it can not possibly be allowed to consume it, thus allowing it to enter our inner bodies.
4) But not all haram things are impure
Like poison and other harmful substances which are haram to consume but not are not impure substances.
Allah knows best.
*I added this part to help with the understanding of the passage.