If you can see through another creature's eyes (such as a familiar or projected image), can you cast a spell which specifies "a creature you can see in range" on a creature you see through another creatures's eyes?
It depends on the specific spell and the way that it is worded.
If the spell requires a target "that you can see" it doesn't matter how you see it so using another creature's eyes would qualify as would scrying magic.
However, you are still bound by the other constraints of the spell. These include:
- Range (PHB p.201): This is measured from the caster, not the point of view,
- A Clear Path to the Target (PHB p.204): The default for all spells is there must be a clear path from the caster to the target.
- Action: Many effects that allow you to use another creature's eyes involve using your action. This limits your spell to bonus action or reaction casting time spells unless you have another action available (e.g. Haste).