我第一次读到弗朗西斯·克里克的《惊人的假说》是在 1994 年，当时我还是个心灰意冷的本科生。我知道我想从化学工程专业转专业，但我不确定自己想转到什么专业。在克里克的书中，我找到了答案。事实上，30 年后，我仍然可以凭记忆背诵《惊人的假说》中的关键句子：“你、你的欢乐和悲伤、你的记忆和野心、你的个人身份和自由意志，实际上只不过是大量神经细胞及其相关分子的行为。”虽然我并不完全相信这个假说是正确的，但我可以把它作为职业来研究的想法让我大开眼界。我仍然保留着克里克这本书的原版，还有十几张便签，标记了最重要的段落。

我最终研究的是视觉和记忆，而不是意识本身，但那本破旧的书一直对我的职业生涯产生着影响。最近一次读这本书是在几年前，当时我正在考虑自己写一本书。虽然我仍然非常尊重克里克这本书的才华，但最近重读时，我震惊地发现它已经过时了。我认为这不是控诉，而是我们这个领域发展的证据。《惊人的假设》反映了 20 世纪 90 年代神经科学的一个时代，在这个时代，研究人员转向以更机械和科学可证伪的方式思考大脑和思维的奇迹。这种方法倾向于将每个现象简化为一个简单的解释，例如基因的表达或大脑区域的活动。例如，克里克提出，我们做出的决定可能不是自由决定的，而是预先确定的，我们对自由意志的幻想可能源于前扣带皮层发生的事情。

现代的替代方案是什么？凯文·米切尔 (Kevin Mitchell ) 于 2023 年出版了《自由人：进化如何赋予我们自由意志》一书，该书反映了一种转变，即将大脑视为一个复杂的动态系统，其新兴属性无法简化为简单元素。在《自由人》一书中，米切尔阐述了真正自由的意志如何存在于一个动态大脑中，在这个大脑中，主体已经进化了数百万年。（《Transmitter》去年发表了《自由人》的摘录。）

米切尔提议的要点是，更高层次的精神状态不仅仅是从神经元的激活及其相互作用中产生的，它们还会影响大脑活动未来的发展方式。在米切尔的解释中，噪音允许多种可能的未来大脑状态，而自上而下的因果影响决定了哪种状态会发生。正是这个过程创造了自由意志。长期以来，对这种观点的反对意见是，精神状态不能既从大脑活动中产生，又同时导致大脑活动，因为在这种论点中，因果关系是“循环的”。米切尔论点的核心是，自上而下的影响不会立即起作用，而是塑造系统的未来，就像一个随着时间展开的“螺旋”。这个发人深省的提议激发了我思考如何测试关于“螺旋式”因果关系的想法——不仅解释自由意志，还解释诸如观看、记忆和感觉等功能。

我非常期待 30 年后重温米切尔的书，看看它是否经得起时间的考验。我希望我们要么已经证实该框架是正确的，要么（就像克里克的书一样）将其视为开创性的但过时的，我们将转向更好的替代方案。无论是哪种情况，都将反映出我们领域的巨大进步。我迫不及待地想知道答案。

I first read Francis Crick’s “The Astonishing Hypothesis,” published in 1994, as a disenchanted undergraduate student. I knew that I wanted to change my major from chemical engineering, but I was unsure about what I wanted to switch to. In Crick’s book, I found the answer in spades. In fact, 30 years later I can still recite key lines from “The Astonishing Hypothesis” from memory: “You, your joys and your sorrows, your memories and your ambitions, your sense of personal identity and free will, are in fact no more than the behavior of a vast assembly of nerve cells and their associated molecules.” Though I wasn’t completely convinced that the hypothesis was correct, the notion that I could make a career out of studying it hit me as a profound insight. I still have my original copy of Crick’s book, complete with a dozen or so Post-it notes marking the most important passages.

I eventually ended up studying vision and memory rather than consciousness per se, but that tattered copy has continued to be influential across my career. I read it most recently a few years ago, when I was contemplating writing a book myself. Though I still very much respect the brilliance of Crick’s book, what struck me on that recent reread was how outdated it had become. I view that not as an indictment but as evidence of the evolution of our field. “The Astonishing Hypothesis” reflects an era of 1990s neuroscience in which researchers shifted toward thinking about the wonders of the brain and mind in ways that were more mechanistic and scientifically falsifiable. That approach tended to try to reduce every phenomenon to a simple explanation, such as the expression of a gene or the activity of a brain area. For instance, Crick proposed that the decisions we make may not be freely decided but instead predetermined, and that our illusion of free will may arise from what happens in the anterior cingulate cortex.

What is the modern alternative? Enter Kevin Mitchell‘s 2023 book “Free Agents: How Evolution Gave Us Free Will,” which reflects a shift toward thinking about the brain as a complex dynamical system with emergent properties that defy reduction to simple elements. In “Free Agents,” Mitchell spells out how will that is truly free could exist in a dynamical brain in which agency has evolved across millions of years. (The Transmitterpublished an excerpt from “Free Agents” last year.)

The gist of Mitchell’s proposal is that higher-level mental states do not simply emerge from the activation of neurons and their interactions, but that they also influence how brain activity will evolve in the future. In Mitchell’s account, noise allows for multiple possible future brain states, and top-down causal influences determine which one happens. It’s this process that creates free will. A long-held objection to this type of idea is that mental states cannot both emerge from brain activity and also simultaneously cause it, because in that argument, causality is “circular.” Central to Mitchell’s argument is that top-down influences do not act instantaneously but instead shape the future of the system, like a “spiral” that unfolds in time. This provocative proposal has inspired me to think about how we might test ideas about “spiraling” causality—to explain not just free will, but also functions such as seeing, remembering and feeling.

I’m very much looking forward to revisiting Mitchell’s book in 30 years to see how well it holds up. My hope is that we will have either verified that the framework is correct, or (as with Crick’s book) we will regard it as pioneering but dated, and we will have moved on to a better alternative. Either would reflect tremendous progress in our field. I can’t wait to find out.