He also wrote a philosophical work called La Monadologie in which he postulated that, (a) since all our knowledge is circumscribed by what we are personally able to understand and observe and (b) our senses and understanding are capable of being deceived, then it is impossible that the entire external world, including other people, may be figments of a deceived imagination.
The Christian philosopher Alvin Plantinga has recently taken up the discussion of the issues Leibniz raised in that essay.
. . . then it is possible that . . .