Over the preceding 20 years, the TARDIS had been in various stages of function, either from barely working to not taking the Doctor to where and when he wanted to go, to outright broken during much of the Third Doctor's run. Finally, during the Sixth Doctor's time, fans finally saw one major reason why that is. After Peri locates the TARDIS manual, the Doctor tosses it aside, knowing what caused their situation and how to fix it. Perhaps if he took the time to read the manual, he could fix the issues with the TARDIS that always seem to plague him (and perhaps see that 6 people are indeed needed to pilot it correctly... a point he knows, but seems to not care about most of the time!).