While mobile and wearable applications leverage biosensing and self-tracking technologies to promote healthy lifestyles through data-based self-reflection, there have been criticisms that they perpetuate the dualistic view that separates mind and body. We examine theoretical premises and design approaches for self-reflection using personal data, and compare them with self-reflection accounts collected from a diary study, in which participants are encouraged to reflect through Focusing on their felt sensations. Based on the analysis of how participants notice, express, question, and respond to their felt senses, we investigate how self-awareness and self-knowledge can be derived from elusive felt sensations and expand the scope of personal data and design for self-reflection. Our findings reveal the gap between theory and practice to design for self-reflection at the limits of biosensing and self-tracking applications, and lead to alternative design propositions for harnessing human senses as personal data for self-reflection.