Law of Proximity
The gestalt principle that elements placed close together are perceived as belonging to one group.
In depth
Proximity overrides almost every other grouping cue, which is why an unrelated object near the subject becomes visually attached to it. It also means grouping can be created purely by camera position — moving so that two distant elements overlap makes them read as one thing, for better or worse.
Examples in practice
A lamp post that appears to sprout from a subject's head is read as part of them because proximity groups the two.