Koshin Pillar on the Old Tokaido 庚申塔
For those interested, one can read about Koshin in many places, but I will give a quick summary that I wrote for another image of mine:
Koshin could be described in English as a "Folk Religion." The core belief is that every person has 3 worms residing in his or her body. These worms keep a record of Bad and Good Deeds for a period of 60 days. On the 60th night, the Day of the Monkey, they leave the person's body to report to Tentei (God), who will then review these deeds and pass a fitting judgment on the person.
There are 3 levels of punishments for those that have been acting badly. If You have Bad Deeds on Your record, You will either become sick, have Your life span shortened or You will Die.
Of course, people don't want any of this to happen, particularly if they've been up to no good. Therefore, to stop the worms from going to Tentei, there are 2 options: First, stay up all night every 60th night so the worms cannot leave Your body (which many people did and some still do, usually partying) or Secondly, protecting Yourself with an image of Shomen-Kongo and the 3 wise monkeys.
Shomen-Kongo is a Deity that protects against disease and of course the 3 wise monkeys don't see, hear or say anything bad about what You've been doing.
The Three Monkeys originated in Japan to replace the three worms and employ a kind of Japanese word play and ideogram. The word for Monkey in Japanese is "Saru (さる)." One ending of old Japanese words that means "not" is the ending "Zaru (ざる)." The Three of them would then be See no Evil, Hear no Evil and Speak no Evil, or 見ざる、聞かざる、言わざる. This also sounds like "Seeing Monkey, Hearing Monkey and Speaking Monkey," however each one is covering his eyes, ears and mouth; accordingly. Thus, a play on words was begun and the Three Wise Monkeys became quite popular until the Meiji Restoration called for a separation of Shinto and Buddhism. This resulted in many Pillars being destroyed. Luckily, there were many believers that hid the Pillars and many have now been collected in groups along roadways and at Jinja (Particularly Hie Jinja, whose messenger is the Monkey) and have enjoyed a kind of revival in belief in some areas.