Everything is causally connected with everything else, directly or indirectly. Being causally connected involves being connected by the relation of logical necessity. Therefore, everything that exists is related internally to everything else, so that without this relation it would not be what it is.
Causality involves intelligible necessity in every case of inference. But can we say the same of causality generally? The principle can be extended to other mental processes, even though the presence of necessity is less discernible. But is there any evidence of intelligible connection among causal processes in physical nature?
There is nothing in causality but regularity of sequence or functional dependence. Between cause and effect there is some kind of intrinsic connection, and this is confirmed by the fact that ordinary inductive procedure involves an argument which without this intrinsic connection would be invalid.
This intrinsic connection is necessary, because when anything is said to have a consequent or a consequence because of its special nature, necessity is part of our meaning, and what follows can't be denied without self-contradiction. This is not to say that we can see why a specific hammer-stroke drives a particular nail. And without some core principle for this, the principle on which all practice is conducted and on which all causal argument is based, would be illusory.