因 yīn shows a person lying inside an enclosure. The outer 囗 (wéi, an enclosed space) frames a 大 (dà, a person with arms and legs outstretched) resting within it. The early reading is concrete: a person stretched out on a mat or bed, the thing one lies upon. From "the mat one rests on" came the sense of a support, a basis, the ground one leans on. That is the hinge of the whole character: what you rely on becomes what a thing rests upon, and what a thing rests upon is its cause.

Three senses branch from the figure on the mat:

1. Cause, reason. The basis behind an outcome, what a result rests on: 原因 yuányīn (cause; reason), 因素 yīnsù (factor), 起因 qǐyīn (the cause that starts a thing).
2. Because of, due to. Used as a connective to name the reason for something: 因为 yīnwèi (because), 因此 yīncǐ (therefore; for this reason).
3. To follow, to go along with. The older sense of leaning on a given ground: to take an existing thing as one's basis and act accordingly, as in 因地制宜 yīn dì zhì yí (adapt measures to local conditions).

The thread is the act of resting upon. A mat is what the body rests on; a basis is what an action rests on; a cause is what an effect rests on. Whether 因 names the reason, the connective that points to it, or the act of following an existing ground, it keeps the same image: one thing leaning on another that came before it.

The busiest modern job of 因 is as a connective that names a reason. 因为 yīnwèi introduces the cause clause and pairs naturally with 所以 suǒyǐ in the result clause: 因为下雨，所以我们没去 "because it rained, we didn't go." The cause comes first, marked by 因为; the result follows, often marked by 所以. Either marker can be dropped once the relationship is clear.

The mirror image is 因此 yīncǐ, literally "because of this," which sits at the head of the result clause and points back at a cause already stated: 他病了，因此没来上班 "he was ill, and therefore did not come to work." Its close relative 因而 yīn'ér works the same way in more written registers. Where 因为 looks forward to a cause it is about to give, 因此 and 因而 look back at a cause already given and draw the conclusion. The literary 由于 yóuyú overlaps with 因为 but leans formal and prefers the front of a sentence.

因 has a natural partner in 果 guǒ, "fruit, result." Together they name the whole arc from cause to consequence: 因果 yīnguǒ, cause and effect. The image is agricultural and apt: 因 is the seed-ground you rest a thing on, 果 is the fruit that grows out of it. What you plant in the 因 is what you reap in the 果.

The pair carries weight in Buddhist thought, where 因果 names the law of karma: every deed is a cause that ripens into a result, and 种因得果 "plant the cause, gain the fruit" captures the moral physics of action and consequence. In ordinary speech 因果关系 yīnguǒ guānxì simply means "a causal relationship," the link a scientist or a detective tries to establish. The four-character 前因后果 qián yīn hòu guǒ stretches the frame across time: the causes before and the effects after, the full story from start to finish.

See the enclosure 囗 as a mat, and the 大 inside it as a person stretched out, resting upon it. That is the literal picture: a body lying on what supports it. Hold the idea of resting-upon and every sense of 因 follows from it.

What the body rests on is a support; what an action rests on is its basis; what an effect rests on is its cause (原因). When you point at that cause to explain a result, 因 becomes the connective: because (因为), and therefore (因此). And when you take an existing ground and build on it instead of fighting it, 因 is to follow, as in adapting to local conditions (因地制宜). Plant the cause and you reap the fruit, 果: that is the whole 因果 arc, from the mat at the bottom to the harvest at the end.