What are our natural principles but principles of custom? In children they are those which they have received from the habits of their fathers, as hunting in animals. A different custom will cause different natural principles. This is seen in experience; and if there are some natural principles ineradicable by custom, there are also some customs opposed to nature, ineradicable by nature, or by a second custom. This depends on disposition.
93
Parents fear lest the natural love of their children may fade away. What kind of nature is that which is subject to decay? Custom is a second nature which destroys the former.[58] But what is nature? For is custom not natural? I am much afraid that nature is itself only a first custom, as custom is a second nature.
94
The nature of man is wholly natural, omne animal.[59]
There is nothing he may not make natural; there is nothing natural he may not lose.
95
Memory, joy, are intuitions; and even mathematical propositions become intuitions, for education produces natural intuitions, and natural intuitions are erased by education.
96
When we are accustomed to use bad reasons for proving natural effects, we are not willing to receive good reasons when they are discovered. An example may be given from the circulation of the blood as a reason why the vein swells below the ligature.