What does it mean "to see" when casting magic missile?

Question

One of my characters is a 1st level magic user who has a bat as a familiar. Being somewhat squishy, I would like to occasionally use fog cloud to hide myself during combat. Since fog provides heavy obscurement which can make the character effectively blind, I am wondering what the limits of "to see" are when casting magic missile. Does it literally mean "I have to visually see who I am shooting as I shoot" or does it mean more generally, "I have to know where my target is"?

Exploring that boundary:

1. Can I cast magic missile from the edge of the fog? Would I be obscured and my enemy visible to me?

2. I cast a mage hand that grabs a hold of my enemy while I fade back into the fog. Does the mage hand now provide me with enough direction to cast magic missile and hit?

3. If my enemy comes into the fog, could I use my bat's blindsense one round to know exactly where he is and the next round cast magic missile?

I know at 1st level I only get 2 slots but as I advance I will have more to use. I also know that it is more efficient to simply attack each round, however I feel my character knows he will die if hit, so will want to avoid that as much as possible. I also think it looks cool to create a large fogbank and cast spells out of it.

Answer 1

It literally means that you have to visually see the target as you cast the spell. It says what it means and means what it says.

Either you're inside the fog cloud, and can't see or be seen, or you're outside of it, and can see and be seen. What you can do is use part of your movement to move out of the fog cloud, cast Magic Missile, then use the rest of your movement to move back into the fog cloud.

Mage hand can't grasp a creature.

The spell says "that you can see," not "that anyone can see," so your familiar seeing a creature does not make it a valid target for your Magic Missile. This would work for a touch range spell that you cast through your familiar that requires the caster to see the target, since the familiar becomes the caster (as far as targeting is concerned). Another way to make use of your familiar would be to use your action to see through your familiar's senses and use another action to cast Magic Missile, although this would require multiclassing two levels into Fighter to get Action Surge or two levels into Sorcerer to get Quicken Spell. Either would be a significant investment, but would allow you to be the one seeing the target on the same turn you cast the spell.

Answer 2

You Must SEE the target

5e uses plain language in the wording and the requirement "to see" requires Sight to do so.

Fog Cloud creates an area that is Heavily Obscured with Fog which (PHB, PP183)

...blocks vision entirely...[characters]effectively suffer from the blinded condition.

To answer your Exploring the Boundary questions specifically:

1. You are in the fog cloud (effectively blinded) - you can not see out of it, nor can an enemy see in. Since you can't see, you can't target with magic missile. Truesight doesn't allow you to see through physical barriers, only magical occlusions. Blindsight should be sufficient unless the target is hidden behind a physical obstruction.

2. No, the mage hand 'touching' the creature isn't enough. However, if you've stepped out of the fog in order to cast/direct mage hand, you could step out of the fog into normal sight, cast magic missile, then step back into the cloud in your turn (obscuring you), as long as you have the movement.

3. It takes an action in order to "See" (via blindsight) through your familiar and it only lasts until the start of our next turn. Unless you have another action to cast a spell, there is no way to combine it's sight with your own spell casting. The only exception is that you can deliver Touch spell attacks through your familiar. From Find Familiar PHB pp 240:

   Additionally, as an action, you can see through your familiar's eyes and hear what it hears until the start of your next turn, gaining the benefits of any special senses that the familiar has. During this time, you are deaf and blind with regard to your own senses...Finally, when you cast a spell with a range of touch, your familiar can deliver the spell as if it had cast the spell. Your familiar must be within 100 feet of you, and it must use its reaction to deliver the spell when you cast it. If the spell requires an attack roll, you use your attack modifier for the roll.

Answer 3

If you cannot see your target, you cannot cast Magic Missile

You are asking a lot of questions in this post, but I will try my best to address them.

What does it mean to see?

For this question we can reference Wikipedia, or Wiktionary(italicized text is marked by me):

Seeing: Having vision, not blind.

Vision: The sense or ability of sight, or something seen, an object perceived visually.

All points seem to lead that "seeing" an object is having a direct visual on it.

Using these definitions for the remainder of the answer, and on the subject of magic missile:

You create three glowing darts of magical force. Each dart hits a creature of your choice that you can see within range.

This implies, using the definitions above, that you cannot just know where your target is. You must have a visual on them. Let's look at Fog Cloud:

You create a 20-foot-radius sphere of fog centered on a point within range. The sphere spreads around corners, and its area is heavily obscured.

Being inside the fog implies that your vision is heavily obscured. From Adam's comment, you are under the effects of blind while you are in the fog.

Blind:

A blinded creature can’t see and automatically fails any ability check that requires sight.

All of this means that you should not be able to use magic missiles in the fog, because you are blind! You cannot physically see the target, even though you may have a rough idea on where your target is.

Think of a scenario that you are behind a wall, and you know for a fact that your target is around the corner. While you know where the enemy is, you can't see them. Magic Missile requires vision. You wouldn't be able to shoot Magic Missile around the corner. What you are asking in your questions is the same thing, just substitute "Fog Cloud" with "wall".

tl;dr: RAW, you should not be able to cast Magic Missile due to the fog blinding you.

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Subquestion 1: Firing from the edge of the fog

Can I cast magic missile from the edge of the fog? Would I be obscured and my enemy visible to me?

See above.

Subquestion 2: Location via Mage Hand

I cast a mage hand that grabs a hold of my enemy while I fade back into the fog. Does the mage hand now provide me with enough direction to cast magic missile and hit?

You cast mage hand, use your action to "grab" your target, and you slip back into the fog. You used up your turn. So what happens next?

Regardless on whether Mage Hand would be advantageous or not, you are still "heavily obscured" (read: blinded) by the fog.

tl;dr: Mage Hand isn't a factor for whether you can cast Magic Missile or not in the Fog Cloud.

Subquestion 3: Sensing via Bat's blindsense

If my enemy comes into the fog, could I use my bat's blindsense one round to know exactly where he is and the next round cast magic missile?

This is a tough call. Let's look at

Blindsight:

A monster with blindsight can perceive its surroundings without relying on sight, within a specific radius.

From NautArch's reference to Crawford's ruling on Sage Advice, the wording "spot" is used instead of "see". Using this information, I don't believe even using Blindsight would allow you to cast Magic Missile.

tl;dr: RAW, you probably wouldn't be able to cast Magic Missile.

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Final notes:

• RAW, you should not be able to cast Magic Missile whatsoever while you are in a Fog Cloud.
• You can always walk out of the fog, shoot a Magic Missile, and come back into the fog.