May One Think In Learning on Tisha Be’av?

On Tisha B’Av, Reb Chaim [Ed., Soloveichik] would not restrain himself from thinking in learning.  In explanation, Reb Chaim would point out that the words the Mechaber writes in Shulchan Aruch (554:3) are, “There are those that forbid learning [even] in thought.” Said Reb Chaim, “If the Mechaber wrote “there are those that forbid”, it’s an indication that there are also those that permit it.”