In Prayer What Is Meant By "Simplicity of Heart"?

Be mindful of the Lord in goodness and seek Him in simplicity of heart; for He is found with those who do not tempt Him, and appears to those who are not unfaithful to him. Wisdom 1:1 - 2

Elder Porphyrios tells a story about a converstion on prayer he had with a visiting Bishop.

He asked the Bishop, "What is meant by praying 'in simplicity of heart and artlessness?'"

The Bishop replied, "Praying with simplicity."

The elder then asked, "And do you understand what that means, your Emenince?"

He responded, "Yes I do."

The Elder then said, "Well, I don't. It is a mystery. It's something that happens only with divine grace."

The Bishop replied, "You are quite right. I don't understand either. An I'm grateful to you for reminding me that simplicity and artlessness can only be understood and achieved though divine grace."

The lesson is that true prayer cannot be gained by any external set of rules or method.  It only comes based on a humble loving relationship with God. Prayer is mystical and comes with grace.

The Elder tells us,

Simplicity and gentleness are a very saintly mode of spiritual life, but you cannot learn this in an external way. It must suffuse itself mystically within you so that your soul embraces this mode of life through the grace of God.

Reference: Wounded By Love, p 118