Is $f(x) = -1.5$ a negligible function? Is $f(x) = -1.5$ a negligible function? Since it satisfies the condition for a negligible function?
You could probably have answered your own question with a quick look at the Wikipedia definition: a function $\mu(\cdot)$ is negligible if for every positive $c$, there is an integer $N_c$ such that for all integers $x &gt; N_c$, $|\mu(x)| &lt; 1/x^c$. The function $f : x \mapsto -1.5$ is clearly non-negligible according to this definition.

Moreover, even without looking at the exact definition, if you think of the (cryptographic) purpose of a negligible function (bounding the probability that an adversary does something he should not be able to do), it would have made no sense to define it so that any negative function is negligible. One can therefore guess that cryptographers try to use definitions that make sense :)