This paper contrasts two approaches to startups: the lean startup and the theory-based view. The lean startup movement has influenced how entrepreneurs approach startups and new ventures, emphasizing the need for a scientific approach. The theory-based view shares this agenda but differs in its theoretical mechanisms and practical prescriptions. The theory-based view focuses on contrarian beliefs, problem formulation, and the development of a startup-specific causal logic for experimentation, resource acquisition, and problem solving. The right mix of entrepreneurial actions is contingent and startup-specific—guided by a startup's unique theory. The paper discusses the differences between the lean and theory-based views, as well as opportunities for partial reconciliation and empirical comparison. It emphasizes that a scientific method for startups needs to recognize the importance of contingent, discriminating alignment between entrepreneurial theories and the actions they prescribe. The paper also highlights the importance of theories in guiding entrepreneurial decision-making and the need for startups to develop their own causal logic for creating value. The paper contrasts the lean startup's focus on customer feedback and bounded rationality with the theory-based view's focus on generative rationality and belief asymmetry. It argues that the theory-based view provides a more comprehensive and flexible framework for understanding and guiding startup activities. The paper concludes by emphasizing the need for startups to pursue a discriminating alignment between the type of theory entrepreneurs seek to explore and the downstream actions or choices related to different forms of experimentation and organization.This paper contrasts two approaches to startups: the lean startup and the theory-based view. The lean startup movement has influenced how entrepreneurs approach startups and new ventures, emphasizing the need for a scientific approach. The theory-based view shares this agenda but differs in its theoretical mechanisms and practical prescriptions. The theory-based view focuses on contrarian beliefs, problem formulation, and the development of a startup-specific causal logic for experimentation, resource acquisition, and problem solving. The right mix of entrepreneurial actions is contingent and startup-specific—guided by a startup's unique theory. The paper discusses the differences between the lean and theory-based views, as well as opportunities for partial reconciliation and empirical comparison. It emphasizes that a scientific method for startups needs to recognize the importance of contingent, discriminating alignment between entrepreneurial theories and the actions they prescribe. The paper also highlights the importance of theories in guiding entrepreneurial decision-making and the need for startups to develop their own causal logic for creating value. The paper contrasts the lean startup's focus on customer feedback and bounded rationality with the theory-based view's focus on generative rationality and belief asymmetry. It argues that the theory-based view provides a more comprehensive and flexible framework for understanding and guiding startup activities. The paper concludes by emphasizing the need for startups to pursue a discriminating alignment between the type of theory entrepreneurs seek to explore and the downstream actions or choices related to different forms of experimentation and organization.