وهذه الأغراض الأربعة تقتضي أن يكون وجه الشبه في المشبه به أتم، وهو به أشهر
These four purposes require the common quality to be more complete and familiar in the comparison term.
Proving possibility, explaining a condition, measuring a degree and reinforcing a condition all work by using the comparison term as a clearer reference point. If that reference is itself obscure or weaker for the intended respect, the explanation loses force.
Being more familiar does not require the comparison term to be objectively superior in every aspect. It means that the relevant respect of resemblance is better established in the audience's understanding; particular purposes such as measuring degree also make the definiteness or fullness of that respect especially important.