There may be fairies at the bottom of the garden. There is no evidence for it, but you can't prove that there aren't any, so shouldn't we be agnostic with respect to fairies?
“The issues and challenges surrounding nuclear non-proliferation are continuously evolving. They've changed dramatically at several junctures in recent memory.”
“Electricity is of two kinds, positive and negative. The difference is, I presume, that one comes a little more expensive, but is more durable; the other is a cheaper thing, but the moths get into it.”