You simpletons, understand cleverness; you fools, understand with the heart. Listen, for my words merit serious attention (Proverbs 8:5-6).
The simpletons and fools to whom Solomon refers repeatedly in Proverbs are not people born without intellect. Why would he demand understanding from those who cannot understand? Solomon is calling out to people who do have the capacity for wisdom, but who choose to behave foolishly.
People who have a limited amount of money, yet squander it on unnecessary incidentals and leave themselves without the means to buy food and clothing, are not necessarily feeble-minded. Rather, they place the pleasures of the moment above the more important things in life. These people allow themselves to be dominated by their desires rather than using their judgment. They do not lack the ability to make a proper judgment, but are lax in applying that ability.
Solomon is speaking to everyone. Few people are as wise as they can be. How often have we regretted doing something, yet we fail to learn from the experience and end up repeating the regrettable behavior?
We would be deeply insulted if someone called us fools or simpletons. We should have sufficient pride not to insult ourselves by behaving in a manner that would warrant such epithets.