What makes you you? I have dark hair, pale skin, and Asiatic features. I wear glasses. My facial structure is vaguely round, with extra subcutaneous fat in my cheeks compared to my peers. What I have done is describe the features of my face. Each of these features described can be thought of as a point within a probability continuum. What is the probability of having dark hair? Among my friends, dark hair is a very common feature, and so are glasses (a remarkable statistic is out of the 300 people or so I polled on my Facebook page, 281 of them require prescription glasses). The epicanthic folds of my eyes are probably less common, as is the extra subcutaneous fat in my cheeks.
Why am I bringing up my facial features in a chapter about spam classification? It's because the principles are the same. If I show you a photo of a human face...
