釋禪波羅蜜次第法門 5.148

5.148

答曰：實爾。快樂樂者，樂定初發，未遍身也。受樂樂者，樂既增長，遍身受。譬如石中之泉，從內涌出，盈流於外，遍滿溝渠。三禪之樂亦復如是。

Answer: It is indeed so. 'Joyful bliss' refers to the bliss absorption when it first manifests and has not yet permeated the body. 'Experiencing bliss' refers to when bliss has already increased and is experienced throughout the body. For example, a spring amidst rocks wells forth from within, overflows to the outside, and fills all the channels. The bliss of the third dhyāna is also like this.