How do I precisely understand 坐井而观天，曰天小者，非天小也 (the origin of the chengyu 坐井观天)?

Question

CC-CEDICT: 坐井观天 (zuò​jǐng​guān​tiān) lit. to view the sky from the bottom of a well (idiom); ignorant and narrow-minded

The origin of the above 成语 (idiom) is reported to be the line

坐井而观天，曰天小者，非天小也。

This comes from the Tang dynasty essay 《原道》 by 韩愈. I understand the above is Classical Chinese, which is new to me. Here's my attempt at understanding this sentence:

坐井而观天 to sit in a well and observe the sky
曰天小者 and declare 曰 the sky is a small thing 小者
非天小也 [but it is] not 非 the sky which is small

The 也 at the end seems to be the special character:

CC-CEDICT: 也 (yě​) (in Classical Chinese) final particle implying affirmation

Honestly, my understanding of this sentence is fairly flimsy, especially 曰, 小者, and 非.

Question: How do I precisely understand 坐井而观天，曰天小者，非天小也?

Answer 1

You already have a good understanding on this idiom, with the words explained, you can help yourself.

坐井而观天，曰(說)天小者(人)，非(不是)天小也(啊)!

Note 曰天小者 are four separate words - "say", "sky", "small", and "person". There is no connection between the individual word like 天小, and 小者, but "天" "小" and "者".

In ancient Chinese writings, you will see many sentences end in one of these 4 letters - 之, 乎, 者, 也. In the past, they are very important for a person's education/knowledge level was exclusively judged by the proper use/handle of those letters. However, the meaning and use of these letters are not an easy task, and a lot of time people have to guess, either by reading the article several times, or using inference techniques. The difficulty in learning old writing style had been the barrier to the vast majority of Chinese in gaining an education, thus, the writing style that included using these letters was abolished right after overthrew of the Qing Dynasty.

In here, my interpretation 也 as 啊 may draw some objections, read on :)

Answer 2

井底之蛙

井底之蛙， 是一个汉语成语，意思是指井底的青蛙认为天只有井口那么大。

坐井而观天 if you sit at the bottom of a well and regard the sky
曰天小者 you could say that the sky is small
非天小也 but it is not the sky which is small yeah!

There is however a story about some ancient Egyptian astronomers who would do this at a certain time of year to regard a particular star during the day.

The belief is an old one. Aristotle (384 BCE – 322 BCE) mentioned it in one of his essays, and Chapter 20 of Charles Dickens’ Pickwick Papers begins with it. Ancient or not, the belief doesn’t work in practice.

Croak!

Answer 3

意思是坐在井里看天的人，说天小，并不是真的天小（是因为他就懂这么点，没见过外边的天）。

这个经常用于对某件事有不同的看法，某些人就会片面的认为xxxx，因为以他的阅历就懂这么多，他也只能说出这样的话。

夏虫不可语冰。

你问一个小学生，等他成为首富，他会做什么，他一定会说买吃不完的糖果，因为他就了解这些。