In endeavouring to establish a theory for the explanation of extraordinary phenomena, there are certain rules which should guide us in the thorny and treacherous path of speculation. But these rules readily flow from the train of thought, and if we examine our own minds during their operations, we {6} 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 {7} 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. 2. His next step was to accumulate facts; he found on enquiry that the persons engaged on these farms in milking, possessed an immunity from Small Pox to an extent sufficient to strengthen the value of his proposition. 3. The reasonableness of the application of the facts to the inference is clear from the coincidence that the eruption on the hands of the dairy people bore a striking {8} resemblance to the Small Pox, and as this disease does not usually occur twice in the same individual, the inference was most reasonable that this eruption protected the people from Small Pox. 4. We have but to take the almost universal adoption of vaccination, and its acknowledged prophylactic powers against the propagation of Small Pox to shew the application of our fourth rule.[2]