dreamed of in their philosophy

[jacob123]

Does "their" refer to "heaven and earth" or does it refer to "great masses"?

Spiritualism, by its persistent investigation of psychic phenomena, by its openly-proclaimed insistence that intercommunication between the two worlds is a present-day fact, has brought great masses of our fellow beings to realize that "There are more things in heaven and earth" than had been previously "dreamed of in their philosophy," and have made many of them, as Christian men and women, understand a mighty truth interwoven with religion...

"The History of Spiritualism," by Sir Arthur Conan Doyle

[probus]

Great masses.

By the way, it is an allusion to Shakespeare. "There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy."

[emsr2d2]

Do heaven and earth usually have a philosophy that they follow?

[probus]

It's Horatio who has a philosphy. Heaven and earth have more things.

[GoesStation]

In Shakespeare's days, "philosophy" meant what we now know as science.

[Piscean]

And the philosophy (or science) is not Horatio's. Shakespeare was using the word your in a way it is still used today:

— with little or no meaning almost as an equivalent to the definite article the
your typical teenager

https://www.merriam-webster.com/dictionary/your