关于这个问题，有很多不同的学说。

Guānyú zhè gè wèntí, yǒu hěn duō bùtóng de xuéshuō.

There are many different theories about this issue.

老师先介绍了几位哲学家的主要学说。

Lǎoshī xiān jièshào le jǐ wèi zhéxuéjiā de zhǔyào xuéshuō.

The teacher first introduced the main theories of several philosophers.

这种学说在二十世纪初非常流行。

Zhè zhǒng xuéshuō zài èrshí shìjì chū fēicháng liúxíng.

This theory was very popular in the early twentieth century.