"Very good again," said her father. "Now an apple is, of course, an inanimate object; and therefore it could not move itself, and Sir Isaac Newton thought that he would try to find out what power moved it."
"Well, then," said Lucy; "did he find that the apple fell, because it was forced to fall?"
"Yes," replied her father; "he found that there was some force outside of the apple itself that acted upon it, otherwise it would have remained forever where it was, no matter if it were detached from the tree."
"Would it, indeed?" asked Lucy.
"Yes, without doubt," replied her father, "for there are only two ways in which it could be moved—by its own power of motion, or the power of something else moving it. Now the first power, you know it does not have; so the cause of its motion must be the second."
"But every thing falls to the ground as well as an apple, when there is nothing to keep it up," said Lucy.
"True. There must therefore be some power or force which causes things to fall," said her father.
"And what is it?" asked Lucy.
"If things away from the earth can not move themselves to it," said her father, "there can be no other cause of their falling than that the earth pulls them."
"But," said Lucy, "the earth is no more animate than they are; so how can it pull?"