What does Berkeley mean by the “doctrine of abstract ideas,” and why does he find it dangerous?

In Objection 4 (Section 45, Page 43), Berkeley employs a variation on the traditional quandary, “If a tree falls in the forest and no one is around to hear it, does it make a sound?” Berkeley answers that it does not. Do you believe that Berkeley answers the objection convincingly? Why or why not?

What does Berkeley mean by “substance”? What reason does he give for believing that spirit is the only substance that exists?