 Mill’s method of difference captures an important kind of reasoning that is used frequently in biological experiments. Let’s suppose we want to find out if a newly discovered compound is an antibiotic, i.e., inhibits the growth of certain bacteria. We start by dividing a bacterial cell culture into several aliquots (samples of same size derived from a homogeneous solution). Then, we add to one group of aliquots the suspected antibiotic that is dissolved in phosphate buffer (“treatment”). To the other group, we add only the phosphate buffer (“control”). Then we record bacterial growth in all the samples (e.g., by measuring the increase in optical density as the culture medium clouds up due to the bacteria). This experimental setup makes sure that the treatment and control samples differ only in the presence or absence of the antibiotic, thus ruling out that any observed difference in growth between the treatment and control aliquots is caused not by the suspected antibiotic but by the buffer solution. Let us denote the antibiotic as “A” and the growth inhibition as “W”. Biologists would thus infer from this experiment that A is an antibiotic if W is observed in the samples containing A but not in the samples not containing A.