Proximity

When perceiving objects, we often group nearby figures together.  Conversely, we see figures which are spaced apart from each other as entirely separate from one another. Consider the following example:

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When someone perceives this example, they could perceive it in many ways. The mostly likely way to perceive this is 3 sets of 2 lines.  This happens because the lines are spaced close to each other, so many people begin to group these lines together in their mind.  Someone, could also perceive 6 separate lines, however this is much less likely to occur due to the proximity of each of the lines with respect to one another.