appeared in the forest , except - 접 기 the planet is furthest from the sun , the area described by such a line is greatest for an equal distance traversed ; and then the planet moves slowest . This law may be deduced from the hypothesis of a central force , but not from any other ; the proof , therefore , as Mill says , satisfies the method of Difference . Apparently , to such completeness of demonstration certain conditions are necessary : the possibilities must lie between alternatives , such as A or not - A , or amongst some definite list of cases that may be exhausted , such as equal , greater or less . He whose hypothesis cannot be brought to such a definite issue , must try to appeared in the forest , except refute whatever other hypotheses are offered , and naturally he will attack first the strongest rivals . With this object in view he looks about for a " crucial instance , " that is , an 접 기