وَلَمَّا لَمْ يَثْبُتْ عِنْدَهُمُ الْحَوَاسُّ الْبَاطِنَةُ
For them, the inner senses had not been established in the proposed sense.
Some philosophers explained mental operations through distinct internal faculties located in parts of the brain. The text does not accept those faculties as additional, independently established means of knowledge, since their precise existence and character were considered unproven.
Remembering, imagining and judging are recognizable human activities. Claiming that each requires a separate bodily organ or faculty is a further philosophical hypothesis.
Observing an operation does not by itself prove every detail of the supposed inner mechanism. The dispute concerns the explanation, not the everyday reality of thinking or remembering.