Metaphors Are Like Similes, but Metaphors Are Not Similes
For example, Man is not God, but may be like a god in some ways. And vice-versa.
Also, the Bible is not Truth, however it sometimes appears to be like truth in some areas. That can be confusing for those who confuse metaphors with similes.
All squares are rectangles, however the converse is not true. Rectangles are like squares, in that both have right angles at each corner. However, the lengths of a rectangle's sides are not necessarily equal, as they are in squares.
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All rectangles are square. In so much , as they all have right angles at each intersection.
Rect =Right
Angle = Angle
RectAngle.
Square, as you use it in your sentence, is an adjective, not a noun. It can be used as both parts of speech. Had you used it as a noun in that sentence, it would have read: "All rectangles are squares." The plural form is required.
The English language is full of potential confusion. When a word can be used as multiple parts of speech the chances of confusion are increased. Add multiple definitions of individual words, along with homophones (right, write) and it gets even more complicated in spoken English.
That's why we need English majors like me. 