From the beginning, the Doctor was a mysterious figure. He appeared to be a frail old man and yet was possessed of unexpected reserves of strength and will. An early writers' guide by script editor David Whitaker describes "Doctor Who" as "frail-looking but wiry and tough as an old turkey". He obviously held tremendous knowledge of scientific matters, and yet was unable to pilot his TARDIS time ship reliably; his granddaughter Susan explained this by saying that her grandfather was "a bit forgetful"; but the TARDIS required expert piloting and guidance by the Doctor. Its systems would often break down, especially the navigational systems. This, combined with the fact the TARDIS was actually designed for "six" pilots, would explain the difficulty the Doctor encountered in piloting it correctly. He was abrasive, patronising, and cantankerous towards his human travelling companions, yet shared a deep emotional bond with his granddaughter Susan. He also harboured a streak of ruthlessness, being willing to lie—and in one case attempt to kill—to achieve his goals. Initially, he distrusted his first two human companions, Ian Chesterton and Barbara Wright, who were forced on him. Over time, however, as they shared adventures together, he grew closer to them, and the TARDIS crew came to share almost a family bond. When proven wrong after accusing Ian and Barbara of sabotaging the TARDIS in "The Edge of Destruction", the Doctor humbly apologised, especially after seeing that Barbara had been hurt by his accusations. The Doctor realised he could rely on Ian and Barbara, sealing a friendship which lasts until their eventual departure from the TARDIS, with the relationship between the Doctor and Barbara being mutually respectful and increasingly affectionate.