The Gemara begins Perek Kaitzad Mevorchim by looking for the source to recite
a bracha prior to eating the food of one’s choice. Originally the Gemara thought that the obligation to recite a bracha stems from the passuk 'קדש הלולים לה. This would indicate that the obligation to recite brachot is d’orayta- Biblically mandated. However, the Gemara rejects this source and concludes that brachot are rabbinic in nature and are deduced from סברא, logical inference. The entire world belongs to Hashem as we say in Hallel 'השמים שמים לה. Therefore, we must not derive benefit until we pronounce a bracha and come to a complete realization that the world is not ours. Only after we recite the bracha can we proceed to the conclusion of the passuk - והארץ נתן לבני אדם- Hashem gave land to man.
Rishonim, medieval commentators, disagree regarding the precise nature of the
prohibition to eat without first reciting a bracha. Is the prohibition to eat derived from
the obligation to recite a bracha i.e. since there is a מצוה to make a bracha therefore one who does not recite a bracha is guilty of גזילה, stealing on the rabbinic level. Or do we assume that that the אסור הנאה the prohibition to derive benefit precedes the obligation to recite a bracha?
Rashi adopts the first approach: the obligation to recite a bracha creates the prohibition to derive benefit without a bracha. However, the Ritva follows the ירושלמי and maintains that the ברכה serves as a פדיון and removes the prohibition for the food.
A careful reading of the language of Rashi indicates that there is a מצוה to recite a bracha and consequently the prohibition to eat sets in. Rashi comments since he had benefit he must thank Hashem who created the items(which he enjoyed). Notice how Rashi phrases his comment in the past tense, שנהנה he derived benefit, rather than שיהנה he will derive benefit. Apparently, not only the bracha before we eat but even bracha acharona, the bracha we say after we eat is derived from this same piece of logic. The Ritva however, would say that the bracha is required to remove the prohibition on food. Therefore, our Gemara is only discussing a bracha rishona, not a bracha acharona. Additionally, according to Ritva it is forbidden to benefit from food prior to reciting a bracha. It would follow therefore that one who is in doubt as to whether or not he said a bracha would not be permitted to eat. Until there is a פדיון the food should remain prohibited. The well-known principle of ספק ברכות להקל would not apply. However, according to Rashi the obligation to recite a bracha is a rabbinic מצוה. Like all rabbinic mitzvot one who is in doubt if he performed the mitzva need not repeat it. Therefore in a case of doubt if one recited a bracha, Rashi would not require that the bracha be repeated, but the Ritva would.