Word for 'believing there is always a simple solution'

What is the word for a person who thinks there is always a simple solution to any given problem?

I have heard this term used before in an intellectual debate in the past, but cannot remember what it is.

The word itself may not have been used to actually refer to a type of person, but to the belief itself (I don't remember this detail either). It also may have been a noun or an adjective (probably a noun, imo), for all I remember.

Addendum 1: in the debate, the belief (or the person holding the belief) was presented as silly, a bit irrational, unrealistic

Addendum 2: the problems the debate was focused on primarily were more down-to-earth in their nature, though global problems were not excluded per se.

A good match should be simplistic — ODO

adjective Treating complex issues and problems as if they were much simpler than they really are: "simplistic solutions"

Naive is associated with meanings you are looking for, but is maybe more strongly asserting that the simplicity of the person is due to their ignorance:

(Of a person or action) showing a lack of experience, wisdom, or judgement

(Of a person) natural and unaffected; innocent

Of or denoting art produced in a style which deliberately rejects sophisticated artistic techniques and has a bold directness resembling a child’s work, typically in bright colours with little or no perspective.

(Oxford dict)

Naif (or naïf) is the associated noun.