Can you target someone with a "that you can see" spell if you can only see a hand or a foot?

Question

If you just get a glimpse of someone turning a corner, maybe just a foot, can you cast a spell that says "that you can see"? A situation that came up for me was as follows.

Command:

You speak a one-word command to a creature you can see within range. The target must succeed on a Wisdom saving throw or follow the command on its next turn. The spell has no effect if the target is undead, if it doesn't understand your language, or if your command is directly harmful to it.

A gnome is on my back and I want to cast command to get it to leave. I can only see a hand, but he can hear me. Can I cast command on him? What about skeletons just cresting a sand dune? And can I do a melee attack?

What I'm trying to get at is what it takes to see someone. Do you have to see center mass, a whole body, do they have 3/4 cover if you can only see a hand, even though it is not covered, just behind me?

Answer 1

Total cover blocks spells and attacks

PHB 196:

A target with three-quarters cover has a +5 bonus to AC and Dexterity saving throws. A target has three-quarters cover if about three-quarters of it is covered by an obstacle.

A target with total cover can't be targeted directly by and attack or a spell, although some spells can reach such a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle

Going off of this, as long as a target isn't behind total cover, you should be able to target them.

If I were the DM at the table where this question came up, I would probably require that the caster be able to see a decent bit of the target, like a hand, head, or foot rather than just an eyelash, but that would be up to each individual DM.

A gnome is on my back and I want to cast command to get it to leave. I can only see a hand, but he can hear me. Can I cast command on him? What about skeletons just cresting a sand dune? And can I do a melee attack?

In this case, the gnome is certainly not behind total cover, and should therefore be a possible target.

What about skeletons just cresting a sand dune? And can I do a melee attack?

As long as they aren't behind total cover, you should be able to target them, although they are likely going to benefit from three-quarters cover if you can only see a bit of them.

Answer 2

An eyelash is enough

There are no hidden rules - “that you can see” means just what it means in English. If someone were to hide and ask “Can you see me?”, any amount of them attracts the answer “Yes”.

If you can see any part of them, you can see them. At best, you can only see half of anybody anyway unless you can see both sides at once.

In game terms:

• You can’t see someone if you can’t see, for example, if you are blinded or closing your eyes to avoid a Medusa.

• You can’t see someone if they are heavily obscured or hidden or invisible or not actually there (i.e. what you are seeing is an illusion of them).

• You can’t see someone if something between you and them blocks your line of sight completely (i.e. they have total cover). You also lack a clear path to the target in such cases.

• You can see someone in a mirror or through a window or a scrying device. You don’t have a clear path to the target which stymies most but not all spells.