The crux of causal inference lies in comparison. Therefore, it is absolutely important for a scientist to understand, define, and develop valid counterfactuals, thus leading to valid conclusions, and consequently, valid decisions. In this article, I explain what counterfactual is. — This is my second post on Causal Inference. In my first post, I talk about the basis of causal inference, and how it is used for making business decisions. In this post, I discuss Counterfactual, Validity, and Biases in Causal Inference in detail.