WordNet sense 1 (mentally healthy; free from mental disorder):

sane

WordNet sense 2 (capable of thinking and expressing yourself in a clear and consistent manner):

coherent, logical

WordNet sense 3 (consistent with or based on or using reason):

rational

WordNet sense 4 (transmitting light; able to be seen through with clarity):

crystalline, limpid, pellucid, transparent, crystal clear

WordNet sense 5 (free from cloudiness; allowing light to pass through):

clear

WordNet sense 6 ((of language) transparently clear; easily understandable):

luculent, perspicuous