Isaac Bergman from Brooklyn wrote:
Recently I read (probably in Scientific American, I don't recall) of a newly-developed brain-scanning device which enables a person to control an onscreen cursor with just his thought alone. If research progresses to the point that a person with his thoughts alone can, say, turn on and off lights, would such "thought actions" be allowed on Shabbat? Technically, all he is doing is thinking! Don't forget that every time a thought goes through your mind, electric currents are generated too, and the same goes for every time you move a muscle.
Dear Isaac Bergman,
In general, the Torah forbids all constructive, pre-meditated acts on Shabbat.
Interestingly, the Torah refers to such acts as "thought-actions" (melechet machshevet; Exodus 35:34), the term which you used.
So, for example, it is forbidden to turn on a light since it causes the filament to glow red (or white) hot; and it is even considered "building" a circuit according to some authorities. Similarly, walking into a dark room and completing the circuit not by manual switch but rather by passing an infra-red beam is regarded as the person's action and is forbidden by the Torah.
In both cases, the person did an action which turned on a light, even though in the second case all he is doing is walking. So too, if a light were to be turned on using just the brain, even though no physical act was performed, since the result is in the physical realm, Jewish Law would forbid it on Shabbat.
Regarding currents in our brain and nervous system during normal activity: The Torah does not forbid using your brain, or any other body part, on Shabbat, even though it generates electric currents. Similarly, the Torah allows eating on Shabbat, even though eating causes the break-down and separation of food into usable and unusable components, activity forbidden if done in a laboratory.
It's actually a commandment to eat and enjoy Shabbat, as Moses said to the Jews, "Eat (the manna) today, because today is Shabbat." (Exodus 16:25) In short, natural bodily functions are allowed on Shabbat.
According to Kabbala, doing mitzvot actually creates spiritual worlds -- still, it is permitted to create these worlds on Shabbat!