A letter written on behalf of the Guardian says: “There are no set forms of meditation prescribed in the teachings, no plan, as such, for inner development.” (Prayer, Meditation and the Devotional Live, in Compilation of Compilations, vol 2, no 1771, p240)
However, I think we have an indication of what Abdu’l-Baha thought meditation was. It comes in a talk from Paris Talks, 1979 edition, pages 96-97 (also in Compilations vol 2, 1752). Unfortunately, Paris Talks is not reliable scripture; nevertheless, I think the text is worth considering.
Abdu’l-Baha begins by saying that for everything there is a sign: “the sign of intellect is contemplation and the sign of contemplation is silence”. I think, at this stage, the word “contemplation” refers to a general state of the mind that includes “meditation”. He goes on: contemplation requires silence, because you can’t speak and meditate at the same time. A little further down in the talk, Abdu’l-Baha refers to “reflection and meditation” in the same phrase, suggesting that these activities are equivalent or close to that.
Abdu’l-Baha then says that it is a “fact” that “while you meditate you are speaking with your spirit”. Well, that’s interesting! This indicates that, for Abdu’l-Baha, meditation involves in internal conversation. Based on that, the need for silence and no talking must apply only to the outside world and not to the mind. Abdu’l-Baha illustrates what he means by this speaking activity. “In that state of mind you put certain questions to your spirit and the spirit answers…”
I take from this that Abdu’l-Baha’s concept of meditation refers to the inner conversation a person has with their higher self. It isn’t about emptying the mind completely. It’s about emptying the mind of the outer world and communing with the divine spirit within.