Brachot 21 - Extra Purity for Shema
In the time of Ezra, the Sages wanted to establish an additional level of purity required for one who says Shema, other prayers, and generally studies the Torah. After he had a seminal emission, he was required to immerse in a mikveh, and immediately after that, he would be considered pure and fit to pray and study. The Sages derived this from the phrase " Just as on the day when you stood before your God on Horeb ," that is, just as for receiving the Torah, men had to separate from their wives prior to that, so later, they had to purify themselves before studying or reciting the Torah. Therefore, the rule reads: one who had a seminal emission thinks the words of Shema in his mind, and as far as the blessings before and after it - he does not say them at all.
Do we see from here that thoughts are the same as speech? - No, for if they were, he could say the words, and not just think them. Then, if the words are not as speech, why is he thinking them, since it does not give him the mitzvah of saying the Shema anyway? - So that he would be involved with the same subject as the congregation.
Art: Peder Vilhelm Ilsted - Boy Studying