All of them means "truth; reality". 誠 is read as "まこと", and 実 as "じつ", however 実 can also be read as "まこと". 本当 seems to have an inclination to a thing or fact, rather than a concept, but it also means "truth, reality".

  • 真 is also 「まこと」. – istrasci May 25 '15 at 2:50

Indeed, what is truth? The answer to your question has two parts: one very simple, and another very difficult.

Today, these three characters are used in slightly different contexts:

  • 誠{まこと} is "sincerity," i.e. a basis in a true heart (see below). Such a judgmental word is not heard much these days in Japanese or English. In fact, you most often hear it in Japanese where it is least called for: in a department store they often play the message ご来店{らいてん}いただきまして誠{まこと}にありがとうございます "we are sincerely grateful to you for coming to our store today", even though the recording is not a human and therefore cannot possess sincerity.
  • 実{じつ} is used to mean "in fact," for example 実{じつ}は猫{ねこ}を飼{か}っていない "actually, I don't keep a cat". It slightly more often refers to externally perceived realities.
  • 本当{ほんとう} is used to mean "actually without lying," for example 本当{ほんとう}の話{はなし} "true story", or 田中{たなか}くんが本当{ほんとう}は何{なに}を考{かんが}えているのか "what is Tanaka-kun really thinking?" It slightly more often refers to internal realities. This is also the word that you belt out when someone tells an unbelievable story. ほんと??
  • 本当{ほんとう} is also used to mean "actually without qualifications," for example 本当{ほんとう}の夏{なつ}の暑{あつ}さは梅雨{つゆ}明{あ}けから "the real summer heat will start after the end of the rainy season".

Now, why does Japan have so many different words that all mean such similar things? Like the English "true, real, actual," this is the result of 2000 years of intellectual history.

In Confucianism, the heart that is true to itself is one that is pure of evil and deception. (This should be contrasted with Western morality, where "being true to yourself" often means breaking promises you have made in the past.) The word for this purity/sincerity is 誠. 誠 has always had this deep moral connotation, and in modern Japanese it is not much different than always.

In ancient China, 実 originally meant the "products" of one's estate. This developed into two important meanings by the classical period: 実{み} "fruit," in the same way that "produce" is used for "fruit" in modern English, and 実{じつ} "what's really produced," in the same way that one "produces evidence" in modern English. The revivalist Zhu Xi promoted his brand of Confucianism as 実学 in order to emphasize that he was not only interested in elite poetry and history, but with a practical philosophy that produced fruit, so to speak, for the prosperity and happiness of ordinary people.

When these two words came to Japan, they were both pronounced まこと in the context of reality. まこと is literally just ま+こと "the real/true thing". But over the course of the Edo period, the kun'yomi まこと became more strongly associated with the Confucian idea of sincerity and the kanji 実 with externally verifiable truth, and now people basically only use 実 with its on'yomi じつ. The reason for this seems to be the arrival of Western learning, which focused the concept of 実学 more finely on scientific verification.

This created a void into which 本 stepped. Most Japanese students learn the word 本{もと} as referring to the roots of a tree. From this, ancient China used 本 to mean "beginning" or "original", but it was never used to mean "true" and was never read as まこと until the Edo period. This is when Zhu Xi's philosophy began to percolate into Japan, teaching that human nature is good from the beginning, and the original heart is therefore the sincere and good heart. Paradoxically, this equation of 本=誠, which was not originally believed in Japan, is what led Motoori Norinaga and other national scholars to assume that Japan needed to recover its "original goodness" from Chinese complexity. 本 is no longer pronounced まこと, but it is from this use of 本 that we get the modern 本当, derived from a mishearing of 本途{ほんと} "original [true] way [of being]".

本当の話です。 For reals.

Source: Tomomi Nishida, Makoto no ji kara Edo ga mieru (1998). ISBN 4876396140

  • In the "simple part" of your answer, by the translations you provided, those three words are different ways of saying the same thing, right? Because if a person talks sincerely, this person talks about facts actually not lying. So these words only have slight nuances between them. Would they be interchangeable in most (or some) cases? By the way, great answer. =) – Yuuza May 24 '15 at 19:56
  • 2
    Yes, it's similar to how English "truly, really, actually" mean almost the same thing and are sometimes interchangeable. I tried to give examples that show when you would hear these words being used more specifically in everyday situations. Like, you say 本当の気持ち but you don't say 実の気持ち. – Avery May 25 '15 at 0:15

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