Systems and methods using machine learning for classifying samples, for example medical images such as dermatological images. The method can use contrapositive logic principals. An example of the method includes: receiving a sample; generating, by a first neural network using the sample: a first classification of a positive label versus a negative label; generating, by a second neural network using the sample: a second classification of the negative label versus not the negative label; and generating, by a category classification module using the first classification and the second classification: a category of the sample.