天行健

Heaven, in its motion, gives the idea of  strength.

君子以自强不息

The superior man, in accordance with this, nerves himself to ceaseless activity.

地势坤

The (capacity and sustaining) power of the earth is what is denoted by Kun.

君子以厚德载物

The superior man, in accordance with this, with his large virtue, supports (men and ) things.

《周易 乾卦第一》

《象》曰：天行健，君子以自强不息。

James Legge: It says in the Great Symbolism: Heaven, in its motion, gives the idea of strength. The superior man, in accordance with this, nerves himself to ceaseless activity.  

                                             理雅各（英译）. Book of Changes （周易）. 长沙：湖南出版社，1992

白话：

《象传》说：天体的运行刚健不已，君子因而要求自己不中断奋发上进。

解读：

《象》即《象传》，分为《大象》、《小象》。《大象》解释卦象，附在《彖传》之后；《小象》解释爻象，附在爻辞之后。大象、小象文中都用“《象》曰”表示。在乾卦，《大象》与《小象》没有分列。从坤卦开始，《小象》都附于各爻的爻辞之后。

《大象》的内容，一般都是先从上下两个经卦分析卦象的意义，然后扩展到人事。如乾卦，上下为乾，乾为天，为健，所以说“天行健”，后边即人事“君子以发奋图强”。