Complex sentence: Lewis mildly preferred S2 over the others; the amended modal system of "A Survey" was S3, but it is S4 and S5 that have generated sustained interest, mathematical as well as philosophical, down to the present day.
Split into simpler sentences: Lewis mildly preferred S2 over the others; the amended modal system of "A Survey" was S3. But it is S4 and S5 that have generated sustained interest, mathematical as well as philosophical, down to the present day.