Abstract: B. Libet is a pioneering neuroscientist in the experimental research field of human consciousness and free will, and his distillate of thought of all his life in consciousness studies can be found in his book named Mind time: The temporal factor in consciousness, which was written in his later years. His most well-known work is his experimental research on free will and his interpretation of the experimental results, but the core of all his works consists in the discovery of time factor in consciousness—that is, time-on theory of awareness —and its rich implications. His research on free will has arisen partly from the time-on theory of awareness. B. Libet started his consciousness studies from the fundamental problem of "how are brain activities correlated with conscious experiences or how do they give rise to conscious experiences?" In deepening into the problem, B. Libet expanded his research into main five subject matters, including: (1) epistemological principles in consciousness studies; (2) how to define the phenomenal nature of consciousness; (3) time mechanism of consciousness; (4) free will; (5) a solution to the "hard problem" of consciousness. In terms of the integration and completeness of investigation of consciousness, B. Libet’s works is unique and distinct, especially in the empirical positivity and predictability. For experimental approach to the problem of how to study the relationship between the brain and conscious experience, B. Libet set out two epistemological principles that must be followed: the introspective report as the operational criterion and no a priori rules for mind-brain relationship. Based on the experimental discovery of time factor in consciousness, B. Libet proposed a time-on theory of consciousness, and elucidated the widespread impact of time factor on human mental functions. The time-on theory has three simple components: (1)To produce a conscious awareness of sensory stimulus, appropriate brain activities must proceed for a minimum duration of about 500 msec (when the event is near threshold). (2)The appearance of a conscious experience has an all-or-nothing character. That is, there is no reportable conscious awareness of an event even if the appropriate brain activities persist for as much as 90 percent of the 500 msec required for actual threshold awareness. (3) although when these same brain activities have durations shorter than those required for the production of awareness, they could be involved in producing an unconscious mental function without awareness. Partly based on the time factor in consciousness, B. Libet made a series ingenious experiments of the sequence of freely voluntary act, and gave an account of the results of those experiments. He then made a conclusion that the volitional process is initiated unconsciously, the conscious will could decide to allow the volitional process to go to completion, resulting in the motor act itself, in other words, the conscious will could block or “veto” the process, so that no motor act occurs. B. Libet conceived conscious experience as mental field, proposing a conception of CMF (conscious mental field) so as to address and solve the problem of unity and causality of conscious experiences.

Key words: B. Libet, Consciousness, Theory of time-on, Unconscious detection, Free will, Mental causation

摘要： 里贝特是人类意识和自由意志的实验研究领域的一个卓越的、先驱性的神经科学家。里贝特的意识研究工作涉及如下四个方面：（1）关于意识研究的认识论原则；（2）对意识现象本性的界定；（3）意识机制的时控理论；（4）对自由意志的阐释和有意识的心智场理论。里贝特的意识研究独树一帜，其时控理论具有坚实可信的实验证据，它从时间维度揭示了有意识的主观体验以及无意识的心智功能与神经活动之间的时间机制。

关键词: 里贝特, 意识, 时控理论, 无意识识别, 自由意志, 心智因果作用