The gemara in Brachos (20b) has a machlokes R' Chisda and Ravina whether "hirhur k'dibbur damei" - is thinking the same as speaking with respect to the mitzva of reading keriyas shema. Most Rishonim pasken that hirhur lav k'dubbur damei based on a stama d'gemara in Shabbos 150 which tells us that with respect to the issue of "dabeir davar" (not speaking about business activities on Shabbos) only dibbur but not hirhur is prohibited (this has become part of our zemiros!) Were hirhur the same as dibbur there should be no difference.
Yet, the Rambam in perek 1 of hilchos Brachos paskens that a bracha may be receited even in one's heart (i.e. hirhur k'dibbur), yet the Rambam also paskens (perek 24 of hilchos Shabbos) that dabeir davar applies does not apply to hirhur. Asks the Sha'agas Arye, why does the Rambam pasken hir k'dibbur with respect to brachos but not with respect to shabbos?
How do you attack this question - by contrasting cases or principles (scrool down to the lomdus posts before this if you don't know what I am talking about)? How do you answer for the Rambam?