In this week's Parsha, we learn that Yosef correctly interpreted Paroh's dreams. The Torah states that his interpretations pleased Paroh and all his servants, and that Paroh then invited Yosef to be the controller of his palace, stating that by his word all Egyptians will be ruled, and only Paroh himself, on his throne, will be greater than Yosef (בראשית מ"א:ל"ז-מ').
Professor Nechama Leibowitz discusses the question of exactly why Yosef's words seemed true in the eyes of Paroh and his servants. Why did they appoint him to a high office, ruling over all of Egypt, before they could be sure of the outcome of his interpretations? This question is even stronger in view of the fact that Egyptian law stated that a slave under no circumstances could be appointed to a high position, as noted by Rashi (לפסוק י"ב שם בד"ה עבד).
The question becomes even more complex when we realize that Paroh's magicians also tried to interpret the king's dreams. According to Chazal, the magicians interpreted the dreams to mean that the seven healthy cows signified seven daughters that Paroh would father, and the seven unhealthy cows represented seven daughters that he would bury. The seven plump ears of corn represented seven provinces that Paroh would take over, and the seven withered ears represented seven provinces that would rebel against him. Chazal therefore say that the magicians interpreted the dreams for Paroh, but their interpretations failed to convince him. How, then, could Paroh know that Yosef's interpretations were correct as opposed to those of his own magicians?
The Abarbanel says that Paroh himself experienced one dream while the magicians thought they were two separate ones. This is made clear by the words in the text, "וייקץ פרעה והנה חלם," "and Paroh awoke, and behold it was a dream," using the singular (שם פסוק ז'). Furthermore, Paroh himself clearly regarded both dreams as one, as he says, "I have dreamt a dream and there is none that can interpret it...," again using the singular form (שם פסוק י"ד). Another place in which the Torah suggests the fact that Paroh regarded his dream as one and not two is in his opening statement to Yosef, "And I saw in my dream..." (שם פסוק כ"ב). This explains why the magicians were unable to interpret Paroh's dream, because they analyzed it as two separate dreams.
But we still have the question of how Paroh knew that Yosef's way of interpreting his dream was correct. The Midrash HaGadol answers that just as Paroh saw the dream, he also saw its interpretation; Paroh would therefore immediately know if his magicians could analyze the dream properly or not. Others add on another explanation, stating that Paroh had "forgotten" the interpretation of the dream but once reminded, he "remembered" its meaning. Memory sometimes acts like this, particularly when not much time has passed; as soon as someone reminds one of the details, he will "remember" what he has "forgotten." In this situation, Paroh's own heart was his witness; he therefore was sure that Yosef's interpretation was correct, and he acted accordingly.