This paper proposes Generalized Agent Theory. Grounded in Standard Agent Model, which is constructed from first principles, the theory leads to the formulation of General Model of Agent Evolution and Intelligent Universe Evolution Model. It culminates in the argument that the Universe itself is a dynamically evolving Agent, with Agent constituting its fundamental unit. This new paradigm, termed ‘‘It from Agent,’’ provides a unified analytical framework for fundamental science. Its breakthrough contributions include: 1) providing analyzable and verifiable fundamental definitions for intelligence and consciousness; 2) describing physical, living, and AI systems within a unified framework; 3) explaining the root of the discrepancies among classical mechanics, relativity, and quantum mechanics from the perspective of the observer's varying intelligence level; and 4) offering new interpretations for fundamental scientific issues such as spacetime, indeterminism, and subjectivity/objectivity.