You probably think you are rational. Day to day, you try to “make the best possible use of the information available to you,” as MIT philosopher Brian Hedden PhD ’12 writes in his 2015 book, “Reasons without Persons.” If you think you are rational because of how you plan for the future, and change your beliefs over time, however, Hedden will be skeptical. Circumstances change, and when using reason, he thinks, all that matters is the present. Thus, he also writes: “The requirements of rationality should be impersonal, avoiding reference to the relation of personal identity over time.” This is what Hedden calls “time-slice rationality,” the idea that in applying reason, we exist in little slivers of time and knowledge. There are not permanently reasonable people, just reasonable decisions. “We are temporally extended people with hopefully long lifespans, but we’re made…

“This was largely due to conversations that were happening in the lounge,” Hedden recalls. “The Stata Center has spaces that really encourage collaboration. A lot of people there were talking about puzzles involving probability, epistemology, and decision theory, and I found those things really stimulating, and wound up specializing in those areas. Things wouldn’t be the same if the building were differently arranged.” Advised by MIT professors Caspar Hare, Robert Stalnaker, and Roger White, Hedden wound up writing his doctoral thesis — three papers — on rationality and decision-making. That formed the basis of “Reasons without Persons,” whose title alludes to a famous work by philosopher Derek Parfit. While it might seem unusual to draw to draw a distinction between our past, present, and future selves, Hedden think…