In theory we would always like to predict qualitative responses using  the Bayes classifier. But for real data, we do not know the  conditional distribution of Y given X, and so computing the Bayes  classifier is impossible. Therefore, the Bayes classifier serves as an  unattainable gold standard against which to compare other methods.In a real life situation why we are not able to calculate the Bayes Classifier ? Why can't we calculate the conditional distribution of Y given X ?Can some one explanation with an example?Any help is appreciated. 
Bayes Classification
classification;logistic regression;supervised learning;bayesian networks