Of course this doesn't mean that we're merely pondering a subject, musing or mulling it over. Meditation which involves thought is a structured, orderly discipline. The meditator concentrates upon his subject, mentally circling it, and that concentration leads him into total absorption. In the Republic, for example, Plato demonstrates this advanced Zen technique when he has Socrates engage in a dialogue on the subject of Justice. The Buddha, in the Surangama Sutra, uses the same technique when he inquires into the nature of the Mind.
Structured inquiry is an ancient meditation form.
But the important thing here is not acquiring knowledge about mind or justice or clapping hands. Entering the Nirvanic precincts, the sacred state of samadhi, experiencing that incomparable bliss, is the goal of any spiritual practice. And obviously this isn't the kind of goal the serious practitioner would even want to attempt in a public environment. Prayer and meditation are personal and private endeavors. Every Zen Buddhist knows this as well as every Christian.