# Evaluation of logical expressions

```
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 ?