How we change what others think, feel, believe and do
When grouping items, we will tend to put together those things which are similar.
In images, we group items based on visual attributes such as shape, size, color, saturation, texture and so on.
Grouped items are then assumed to be a single, larger whole.
In the example below, A may well be seen as a single collective shape, as all the sub-parts are circles. B, however, may be divided into two vertical columns, one of circles and the other of triangles.
When we are seeking to understand our environment, we first identify atomic shapes and then cluster them together into larger, nameable objects. Similarity is a simple rule that we use in this.
The basic assumption is that things which are associated by visual similarity also have other similarities and hence belong in the same group.
Similarity also appears as a principle when we are wondering to trust other people. If they are similar to us, for example with similar heritage, we will be more likely to trust them. Confidence tricksters use this principle when they dress and act in similar ways to people we trust, from clerics to leaders.
If you want people to see things as being the same, then use similarity in some way. You can connect things through a wide variety of attributes.
Once you have connected them with real similarities, you may then assume that they have other similarities. This extension may well go unchallenged.