What is abductive reasoning and when is it used? First, it is a kind of inference. In a broad sense, a logical inference is any operation that, starting with some information, allows us to obtain some other information. People are continually doing inference. For example, I cannot remember which of the two keys in my pocket opens the door of my house. I try with the first one but it does not open. So I conclude that it should be the other. In this case, the inference starts with some data (premises): one of the two keys opens the door, but the first I tried did not open. I reach some new information (conclusion): it is the second key.Inference (or reasoning) does not always follow the same way. In the example, if I know that one of the keys opens my house and I cannot open with the first key, it is necessary that the second opens. When this is the case (that is, the conclusion follows necessarily from the premises), then we are facing a deductive inference. Given that the conclusion is a necessary consequence of the premises, there is no doubt that the conclusion is true, whenever premises are all true: the door must be opened with the second key.But there are many contexts in which we cannot apply deductive reasoning. Sometimes, we use it but we become surprised by the outcome. What happens if finally the second key does not open the door? This kind of surprise was studied by the philosopher Charles S. Peirce as the starting point of abductive reasoning: