In the sixteenth and seventeenth century, empiricists such as Pierre Gassendi and John Locke accounted for the human capacity to know about the world by emphasizing sensory access to information. These philosophers entertained skepticist doubts concerning reasoned knowledge and concluded that sure knowledge of the world is won primarily by empirical or sense-based studies. These views on our general knowledge and certainty support a variety of claims concerning scientific knowledge in particular. On the basis of their constructive skepticist stances, these two empiricists support arguments for the reality of scientific objects and a strongly empiricist methodology of gathering and organizing scientific knowledge. The hallmarks of their empiricist philosophies of science are a background essentialism which motivates the search for underlying causes, a probabilism about theory confirmation, and a ranking of knowledge-types by degrees of certainty. The similarities of their views illuminate the influence of Gassendi's writings on Locke. Their differences show us their varying conceptions of knowledge with certainty and of knowledge about the world.