Casting Time: 1 reaction, which you take when you see a creature within 60 feet of you casting a spell
What does "you see" refer to, the creature or the casting of a spell? In other words, can you Counterspell for example, if you hear a creature within 60 feet of you casting a spell, and see the creature?
Example: You are behind someone with full-face helmet, and they cast a verbal-only spell at someone else in front of them. Can you Counterspell?