Foundationalism is a philosophical theory that suggests knowledge is built upon a foundation of certain, indubitable beliefs. These basic beliefs serve as the starting point for constructing further knowledge, much like a building rests on a solid base. Foundationalists argue that some beliefs are self-evident or justified without needing further support. In this framework, beliefs can be categorized into two types: foundational beliefs, which are secure and unquestionable, and non-foundational beliefs, which depend on the foundational ones for their justification. This approach contrasts with other theories, such as coherentism, which posits that beliefs are justified by their coherence with one another rather than a foundational base.