Is there no such thing as species?
24010
How could humans have interbred with Neanderthals if we're a different species?
87475
Why are there only four fundamental interactions of nature?
100178
Time, what is it?
29538
Does a non-abelian semigroup without identity exist?
29772
How is uniq not unique enough that there is also uniq --unique?
40328
Can something be nothing?
9709
Is there a word or phrase for something that one might wish exists, but most certainly doesn't?
79052
How does ecology differ from biology?
113606
Dimensionless quantities
66716
Does it make sense to classify all humans in a single species?
125783
What's this? What is it? but not What's it? - Why?
147211
Why are not all species hermaphrodites?
155955
What on earth has yet to be explored?
58404
Does vacuum (empty space) exist?
42343
Is spacetime all that exists?
138569
Is the “sum of all natural numbers” unique?
56725
Human vs. alien to avoid extinction by omnipotent being