There is simple as in reduced to essentials, plain, homely, frugal, basic, - not complicated.
There is simple as in effective and efficient, economical, functioning well - not cluttered.
There is simple as in self explanatory, understandable, clear, transparent, honest - not obscure.
Simple is often used to mean natural, as nature intended, - not contrived or man made.
Simple can be sophisticated - Japanese gardens, Swedish design, minimal music - an appearance of simplicity, but richly overlaid with meaning and cultural references. Very contrived. Totally man made.
Simple can also, points out Kevin McLeod of Grand Designs 'be hugely expensive'.
Raymond Blanc talks of 'complicated simplicity' - I think he may be referring to the army of staff at his command when he creates a 'simple' dish on his new TV series!
There is also simple minded and the simpleton, not expressions often heard today.
Such a wide range of meanings for one innocent word!
I plan to take a look at what I mean when I'm referring to my own taste and preference for 'simple' in my life, and since the time is right I'll start with my garden....