Evaluation of logical expressions

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sage: foo=sin(x) > 0 ; foo
sin(x) > 0
sage: bar=cos(x)>0 ; bar
cos(x) > 0

I do not understand this :

sage: foo and bar
sin(x) > 0
sage: foo or bar
cos(x) > 0

As long as x has no value, these expressions can't be evaluated or simplified. Could some kind soul enlighten my confused mind ?