Simple Person

What is the noun to express the state of a simple person?

The adjective "simple" might have many meanings.

Some dictionaries say "a simple person is stupid".

 STUPID [not before noun] someone who is simple is not very intelligent

I’m afraid Luke’s a bit simple.

But, the internet also have another definition of "simple person" which is completely opposite the one mentioned above.

Simple people, or people who claim minimalism, simplicity, and easy-going lives, are relaxed, patient, and present in their everyday lives.

Simple Person

A simple person is someone who is uncomplicated. They're grateful for the little things in life. They don't try to impress, they're humble. They know what really matters and what doesn't, they're very creative and of higher intelligence than most people, others have trouble understanding them because of the different levels of intelligence. A Simple person is usually of the highest intellect, great character, experienced, and wise. A simple person has a heart of gold and loves nature, their friends and family. It's the simple things in life that they enjoy like quality time. A simple person knows their time is short, so they'll spend every breathing second to make the best of it. They're great listeners, give the best advice. Smart, charismatic, brave and blessed with natural beauty.

In everyday conversation,

Do people think his is stupid if they hear someone say "he is very simple" or it depends on contexts?

Now, "humility" is the noun of "humble" and we have this sentence

He is very humble and his humility set a good example for his children

Now, suppose I use the adjective "simple" with its positive meaning above "claim minimalism, simplicity, and easy-going lives, are relaxed, patient"

What is the noun of this adjective?

What should I fill in this blank?

He is very simple and his ____ (simplicity/simpleness/...) set a good example for his children

There’s an American hymn called “Simple Gifts,” written by Joseph Brackett in 1848, that’s one of the most famous examples of the sense you found.

'Tis the gift to be simple, ’tis the gift to be free

'Tis the gift to come down where we ought to be,

And when we find ourselves in the place just right,

'Twill be in the valley of love and delight.

When true simplicity is gained,

To bow and to bend we shan’t be ashamed,

To turn, turn will be our delight,

Till by turning, turning we come ’round right

Despite that, calling someone “simple” is almost always an insult, and means they are mentally impaired.

A better word to use might be authenticity, meaning “not false or imitation” and “true to one's own personality, spirit, or character.” Synonyms include honesty, genuineness, sincerity, integrity and trustworthiness.

Simple in the positive sense also connotes a kind of down-to-earth lifestyle, which doesn’t try to impress anyone. The first words that come to mind for it are modesty, unpretentiousness, unaffectedness or naturalness (although this can mean either be true to one’s own nature, or in harmony with the natural world). Any of the negated nouns might be replaced with a phrase such as, “lack of pretensions” or “way of not putting on airs.”

Simple can also imply a rural lifestyle, also called rusticness, rusticity or bucolicity (a word so fancy that using it is ironically doing the opposite)

Another idiom of this (from the Gospels, and with a religious connotation) is, the salt of the earth, “A person or group that is regarded as genuine, unpretentious, and morally sound.” Even Jesus, though, threw in a jibe afterward: “But if the salt has lost its savor, ....”

There is an idiom "a simple life" which it sounds like you are describing. From Merriam-Webster

used to refer to a manner of living in which a person does not own many things or use many modern machines and usually lives in the countryside

One generally "seeks to live a simple life" or "believes in a simple life". While it is negative to "be simple" it is generally positive to "desire simplicity". So your example becomes:

He seeks to live a simple life and this simplicity set a good example for his children