These objects are not recognised in isolation.

We cannot well miss Cleopatra's Needle, if we are in its neighbourhood; but no one has seen a single molecule or a single electron, yet the characters of events are only explicable to us by expressing them in terms of these scientific objects.

Undoubtedly molecules and electrons are abstractions.

But then so is Cleopatra's Needle.

The concrete facts are the events themselves--I have already explained to you that to be an abstraction does not mean that an entity is nothing.