Empirical evidence, data, or knowledge, also known as sense experience, is a collective term for the knowledge or source of knowledge acquired by means of the senses, particularly by observation and experimentation. The term comes from the Greek word for experience, ἐμπειρία (empeiría). After Immanuel Kant, it is common in philosophy to call the knowledge thus gained ''a posteriori'' knowledge. This is contrasted with a priori knowledge, the knowledge accessible from pure reason alone.