Scientists maintain that "a physical action requires a physical cause". However, if I am in deep thought about my dead spouse, for example, and a tear comes to my eye, how is it that the mind (thought) can cause a physiological response (a tear)? Does this not contradict the (physical) law of cause and effect?
The short answer is no, this doesn't violate the law of cause and effect because the mind itself is a physical entity. Your thought experiment hinges on the debate of materialism (the mind is a physical thing) versus dualism (the mind is a different kind of thing than physical things). Most cognitive scientists believe that materialism is the correct view.
I disagree with @Josh (and all the other answers) that a materialist viewpoint is required to resolve this apparent contradiction.
Firstly, we should not infer that the thought caused the tear. As an example, if I were to press the PrintScreen key on my keyboard, then a popup message would appear on my screen saying "printing to deskjet printer" and then shortly after that, the printer would print a copy of this answer. However, the popup preceding the printing does not mean that the message caused the printing. Presumably, pressing the PrintScreen key caused both events.
We might also ask what caused the pressing of the PrintScreen key in the first place. One might then suggest, "well, I thought about pressing the PrintScreen key, and that caused me to press it." But it turns out that this is not likely the case. In his seminal experiment, Benjamin Libet demonstrated that awareness of pressing the key comes after pressing it, not before (and a later replication of this experiment further demonstrated that in fact pressing the key actually causes the thought of pressing it, not the other way around). So we cannot be certain that the thought even preceded the tear, let alone caused it.
Modern theories of emotion suggest that much like decision awareness, emotion is similarly "inferred" from a combination of physiological cues and other context information - that is, first I feel sad, then I infer that the cause of that sadness is the memory of my late spouse. So more likely something else (perhaps a feeling of sadness) caused both the thought and the tear.
The large body of research that was spurred on by Libet's experiment has significantly changed our understanding of the role of "thought" or "will" or "volition" in the causal chain, such that they now appear to be a side-effect rather than cause of physical action. So regardless of whether you prefer a physicalist or dualist view, there is no contradiction here.
This would be analogous to a factory robot mounting a door on a car that is being built. There is no "physical" reason that the untrained eye can see, which compels the robot to do what it does. There is no visible system of weights and pulleys that moves the robot's arm.
In fact all the instructions the robot needs to do its task are encoded on a chip as different voltage levels in some microscopic elements. So there is still a physical cause for what the robot is doing, it is just not immediately obvious to people who aren't engineers.
The same applies to the brain, except that the links between physical cause and effect there are even more unclear.
Your line of reasoning seems to be that
just thinking is not a physical action, a tear is a physical action, so just thinking cannot cause a tear. There is a lot of this naive materialism going on, even in the scientific circles, but this seems to be changing. There are so many things wrong with this argument.
"Just thinking" is a very significant action, a view that, even before modern neuroscience, was embraced e.g. by the Alexander Technique. It can take a lot of energy. For example, when I was at an EyeBody retreat, we used to get very hungry after sitting all day long, try to connect with our eyes. Are you still claiming it is not "material"?
Then what is material? Consider W.V.O.Quine's On What There Is. Things that exist are, well, the ones that we include in our scope of reference. Because if you say that thoughts or feelings are not material, that they don't exist "here", then "where" is it that they exist?
How do you know that the thought about a spouse is what is causing the tear? Correlation does not imply causation. This strikes me as a very simplistic view. As someone with considerable meditation experience, I can suggest that it seems to be more tricky than that.
The concept of a "law of cause and effect", the way you seem to be using it, is itself so simplistic as to almost be useless. What exactly is causality, in your view? For your further research, note that in Hinduism basically the same concept is known as "karma".
As a postscript, this is just a very narrow way of looking at things. This is like saying "science has proved that there is no God", and if someone disagrees, concluding that they are a crazy religious fanatic who does not believe in science. Science hasn't even proved that the total entropy in the Universiy is increasing. Furthermore, the First Law of Newton cannot even be proved, in theory. Here I would suggest e.g. Against method by Paul Feyerabend.
Question: Can the mind affect the physical brain?
"Scientists maintain that "a physical action requires a physical cause". However, if I am in deep thought about my dead spouse, for example, and a tear comes to my eye, how is it that the mind (thought) can cause a physiological response (a tear)? Does this not contradict the (physical) law of cause and effect?"
From the way you ask, it seems that you believe that the mind is a non-physical entity, so when it (such as thought) causes a physiological response (such as a tear), it contradicts the (physical) law that “a physical action requires a physical cause”.
This kind of belief that the mind is a non-physical entity – an entity that is not made up from or is not part of physical entities such as mass, energy, or force and thus cannot react with physical entities with known physical laws – is called Dualism (ref 1, ref 2, ref 3, ref 4). And you’re right, if this is true, it is not possible to explain, at least for now, how the mind can affect the physical brain. Several new hypotheses are needed to account for the nature of the mind, how it can affect the physical brain, how the physical brain can affect it, etc.
Now, if the mind is not a non-physical entity, it must be a physical entity such as mass, energy, or force, but what is it? Because it’s not obvious that the mind is a physical entity (i.e., it’s not obvious that the mind is simply a mass, an energy, a force, etc.), if it is a physical entity, it must be a physical entity in a non-obvious form (otherwise we would have identified what it is already).
The next question is what physical entity in what form is the mind? This is still an inconclusive matter; no unanimously approved answer exists. But one recently emerging interesting concept is like this. If you reflect on your mind, you’ll see that the mind is a highly dynamic, information-intensive entity (because your thoughts, visions, emotions, etc. are always changing, changing fast, and full of information). Also, studies in cognitive neuroscience find that the activities of the mind are always associated with electromagnetic activities, which can be studied in the form of electroencephalography, electrocorticography, intracortical recording, magnetoencephalography, event-related potentials, etc. Moreover, the activities of the mind can be affected by electrical stimulation and magnetic stimulation. So, the entity that is the mind must have these properties, that is, it must be highly dynamic, information-intensive, must be associated with electromagnetic activities, and must be affected by electrical/magnetic stimulation too. (ref 5, ref 6)
It’s obvious that mass, energy, and force in their usual forms do not have these properties. In their usual forms, they certainly are not always highly dynamic, information-intensive, and not all of them are associated with electromagnetic activities and affected by electrical/magnetic stimulation. If you study the brain carefully, you’ll find that the only physical entity in the brain that is always highly dynamic, information-intensive, always associated with electromagnetic activities, and always affected by electrical/magnetic stimulation is the neural signaling of the brain, which ceaselessly sends innumerable electrical/electrochemical signals among billions of nerve cells in the brain.
So, the neural signaling is the entity that is very closely associated with the mind. If no other entity is found to have the above properties of the mind as the neural signaling does (which, indeed, none has been found yet), it’s logical and inescapable to conclude that the mind is, in fact, the neural signaling of the brain. This possibility is strongly supported by the testable fact that anything that affects neural signaling alone (such as electrical/magnetic stimulation, and some selective neuro-pharmacologic agents) can affect the mind and that anything that affects something else but does not affect neural signaling does not have effect on the mind. (ref 5, ref 6)
Now, if the mind is the neural signaling itself, it can naturally affect the physical functions of the brain, without having to create new hypotheses to explain its effects. For example, in your question, if the thought is a neural signaling itself, it will naturally be able to affect other functions of the brain by its signals and finally create a physical response, such as a tear. This is one of the physical explanations of what the mind is, whether it can affect the brain, and how it can affect the brain. It must be remarked here again that, although this concept is interesting and seems logical, it is not the standard view of the mainstream scientific community yet, and other physical explanations exist. You can find them by searching the term physicalism (for example ref 7, ref 8, ref 9, ref 10).
Edited Summary: I add the full info of the references at the end of the answer here so that they can be found in case the links break.
Calef S. Dualism and mind. Internet Encyclopedia of Philosophy. https://www.iep.utm.edu/dualism/
Robinson H. Dualism. Zalta EN, editor. The Stanford Encyclopedia of Philosophy (Fall 2017 Edition). Retrieved 2018 May 02 from https://plato.stanford.edu/entries/dualism/
Zimmerman D. Dualism in the Philosophy of Mind. Encyclopedia of Philosophy. 2nd edition. 2005. p113-122. http://fas-philosophy.rutgers.edu/zimmerman/Dualism.in.Mind.pdf
Wikipedia. Mind–body dualism. https://en.wikipedia.org/wiki/Mind%E2%80%93body_dualism
Ukachoke C. Chapter 1. Theorem I: The Mind is Part of the Functioning Brain. In: The Basic Theory of the Mind. 1st ed, 2018. p 8-18. Charansanitwong Printing Co. Bangkok, Thailand. https://mindtheory.net/new-page-1/
Ukachoke C. Chapter 2. Theorem II: The Mind is the Composite of All Information-processing Processes of the Brain. In: The Basic Theory of the Mind. 1st ed, 2018. p 19-29. Charansanitwong Printing Co. Bangkok, Thailand. https://mindtheory.net/chapter-2/
Stoljar D. Physicalism. Zalta EN, editor. The Stanford Encyclopedia of Philosophy (Winter 2017 Edition). https://plato.stanford.edu/archives/win2017/entries/physicalism
Giannetti E. The possibility of physicalism. Dement Neuropsychol. 2011 Oct-Dec;5(4): 242–250. doi: 10.1590/S1980-57642011DN05040002. PMCID: PMC5619037. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5619037/
Gillett C, Loewer B, editors. Physicalism and its Discontents. Cambridge University Press. 2001. Cambridge, United Kingdom. ISBN 0521801573 hardback. https://www.physicalism.com/physicalism-discontents.pdf
Wikipedia. Physicalism. https://en.wikipedia.org/wiki/Physicalism
The thing is, being in deep thought about something is a physical action on a far smaller scale. If all your neuronal and glial cells died you wouldn't be thinking. These very cells are physical, and thought is caused by physical changes within these cells (ionic movement, neurotransmitter release, etc. ) Without these physical changes on the micro scale of the brain your thoughts would cease to exist, and therefor no tear as a consequence of the thoughts.
Regardless of which side of the materialism vs dualism debate you find yourself on, the result is the same. If the mind is "material" then there is no issue. If the mind is immaterial, then the mind is the same kind of thing as a running software program. In that case, it doesn't have a physical form, but affects the state of the device that is running it. How? As long as you remember not to conflate the binary image of the program's instructions with the running task itself, then it makes sense. A running task is little more than a series of states in the device that is running it. Even the active instructions are nothing more than a collection of states. These state collections are constantly accessed as the result of previous state changes from other state collections in the device. The brain can thus be viewed as an insanely complicated collection of nano-computers running software called "mind".
That the "mind itself is a physical entity",seems to be very controversial,to say the least.In this respect Henry Stapp (a physicist at Berkeley) was asked during a lecture "what is this " I " you are talking about".He answered:"It might be that at some point the mind will be found to be material,but so far there is no evidence whatsoever that that will be the case" Also,when it comes to the so called " the causal closure of Physics" ( every physical effect has a physical cause) is also questioned by a number of scholars,including Stapp,who has comented that it is a remnant of Classical Physics and no longer valid as the observer,and his mind,has entered the picture with the advent of Quantum Mechanics. And relating to materialism,even Einstein remarked: "In modern Physics there is no place for matter and the field,because the only reality is the field". So,as a layman,I am not sure what is the basis of materialism anymore,I would think that after Quantum Mechanics replaced the Newtonian physics there would be no more materialism. I would appreciate it if someone would shine a light into these subjects. Alberto