Chapter 47 of the Tao Te Ching

不bù出chū户hù，知zhī天tiān下xià；不bù窥kuī牖yǒu，见jiàn天tiān道dào。
其qí出chū弥mí远yuǎn，其qí知zhī弥mí近jìn。
是shì以yǐ圣shèng人rén不bù行xíng而ér知zhī，不bù见jiàn而ér名míng，不bù为wéi而ér成chéng。

All men in the universe strive for the same goal, though by different paths. Their feelings do not differ from mine. That is why, without stepping out of my 户 hù (door), I can know the universe; without looking through my 牖 yǒu, I can know the 天道 tiāndào (ways of heaven). What is most important for man lies within him, and therefore very close to him. If he seeks it outside, he moves farther away from it.

苏轼 Sū Shì : Such is the essence of human nature that it embraces and traverses the entire universe; it does not recognize distance or closeness in time or space. The Sage knows all without stepping out of his 户 hù and without opening his 牖 yǒu, for his nature is of perfect completion; but men of the world are blinded by material things, their nature is limited by the bounds of the senses; they are disturbed by their body and heart. Externally, they are stopped by mountains and waters, they see no further than the reach of their eyes, they hear no further than the reach of their ears. Even the flimsiest fence can paralyze the use of these two faculties.

刘克福 Liú Kèfú : The universe is vast. It is necessary to go out to know it. But the space we can traverse with the strength of our feet is an infinitely small thing; what we can know of it is very limited.

The 天道 tiāndào have an immeasurable extent; one must absolutely observe them to judge them; but the distance we can reach with the strength of our sight is very limited; what we can perceive is very meager. The Sage knows the nature of the universe; he understands the 天道 tiāndào, for he possesses everything within himself.

A single commentator understands by 天道 tiāndào the operations of the two principles 阴 Yīn and 阳 Yáng, and the movements of the sun and moon.

Men of the world are blinded by interest and passions. They rush out to satisfy them. The love of profit disturbs their prudence. That is why, day by day, they move farther away from their nature. The dust of passions thickens day by day, and their heart darkens more and more. That is why, the more they distance themselves, the more their knowledge decreases. But the Sage remains calm and without desires; he does not concern himself with sensible things, and, by remaining at rest, he understands all the secrets of the universe.

According to certain texts, the Sage does not need to ascend to heaven or descend into the abyss to know heaven and earth. He knows them through his own heart.