Linguistic relativity, sometimes called the Whorfian hypothesis, posits that properties of language affect the structure and content of thought and thus the way humans perceive reality. A distinction is often made between strong Whorfian views, according to which the categories of thought are determined by language, and weak views, which argue that language influences thought without entirely determining its structure. Each view presupposes that for language to affect thought, the two must in some way be separable. The modern investigation of linguistic relativity began with the contributions of Benjamin Lee Whorf and his mentor, Edward Sapir. Until recently, much experimental work has focused on determining whether any reliable Whorfian effects exist and whether effects truly reflect differences in thought caused by linguistic variation. Many such studies compare speakers of different languages or test subjects at different stages of language acquisition. Other studies explore how language affects cognition by testing prelinguistic infants or nonhuman animals and comparing these groups to children or adults. Significant progress has been made in several domains, including studies of color, number, objects, and space. In many areas, the status of findings is hotly debated.