shall find that the following is the course of our instinctive reflections. It is a course we adopt as the test of theories when formed, and is a guide in all cases for their construction.
We first commence with an idea, which exists in our minds in the form of a proposition: then the following rules naturally suggest themselves:—
1. The probability of the value of our proposition from inference.
2. The number and value of facts to support the proposition.
3. The reasonableness of the application of the facts to the inference.
4. What amount of information in the form of results can be produced in proof of the tenableness of the proposition.[[1]]
In illustration of the value of these rules the history of Dr. Jenner's discovery affords an appropriate example. To use the words of Dr. Gregory, "he appears very early in
life to have had his attention fixed by a popular notion among the peasantry of Gloucestershire, of the existence of an affection in the cow, supposed to afford security against the Small Pox; but he was not successful in convincing his professional brethren of the importance of the idea."
The popular notion of the peasantry originated the idea in Jenner's mind, and it became fixed there as a proposition.
1. He commenced his enquiry by observing that the hands of milkers on the dairy farms were subject to an eruption, and he inferred that the notion of the peasantry bore the stamp of probability, which strengthened the idea in his mind and gave force to the proposition.