Finding one ruling in two different kinds does not automatically show that every property they share is its cause. The sun and fire can both heat things, but not every feature they share explains heat. The objection is that a shared ruling does not, by itself, establish a shared effective reason.
A further question arises: if existence makes vision possible, must everything that exists always be seen? The reply distinguishes possibility from actual occurrence: eligibility for vision is not the same as being seen at every moment.