"Who can understand his errors? Cleanse Thou me from secret faults. Keep back Thy servant also from presumptuous sins; let them not have dominion over me: then shall I be upright, and I shall be innocent from the great transgression." (Psalm 19:12-13, emphasis mine)
I presume
That word presumptuous is "zed" in Hebrew. It means arrogant and its root goes back to "seethe" and "insolence". "Pride". Peter says that God "resisteth the proud" (1 Peter 5:5). If you've ever had a taste of God's true character--of the beauty of His Person and His presence, you'd be open to Him searching out any vestige of "presumption" resident in the recesses of your soul that would keep you from reveling in His fullness as time goes by.
"My soul thirsteth for God, for the living God: when shall I come and appear before God." (Psalm 42:2)
Keep back Thy servant from presumptuous sins...
Just what is a "presumptuous sin"? David prays in the verse prior: "Who can understand his errors?" Not me. I can be inexpressibly thick at times. As I have no window, in and of myself, into what is really going on in another person's heart and mind, my ways of comportment mustn't be based on what they think. They may be open, honest and transparent but what they think and therefore say is not meant to be the guiding rubric for my way of thinking acting and being. The only being who fits that description would be God. And He has given us enough of His heart and mind with which to substantiate our life and conversation in His word. Without His word, without Him expressing a few guidelines and rules and guardrails, we would never have any chance of getting right with Him--and of developing a mind more like His. He also sent Jesus. Jesus is the living word, by the way (see John 1:14). But again, what does a presumptuous sin look like? If I have any presumption as to what one might be, would that in itself be one? As humanity--without Jesus--is irreparably divorced from God the Father any notion as to how to get right with Him again would necessarily be my "own understanding" (Proverbs 3:6) on which Solomon says to "lean not".
"My people are destroyed for lack of knowledge." God is serious here. He continues: "because thou hast rejected knowledge, I will also reject thee..." (Hosea 4:6)
Theory of Mind says that, because we cannot truly know what's going on in another's mind, we formulate our own constructs of interaction based on observation and intuition mixed with past effectiveness. On one level, it totally makes sense but it necessarily denies anything spiritual. I find that people (myself included) are more complex than we know and while another person may be operating in a largely normal way, it does nothing to reveal their true motive for doing things. If we really cared how things panned out, we might be a little more circumspect when choosing friends and acquaintances. Not a bad thing to have lots and lots of friends per se. Just make sure they're truly friendly. No Theory of Mind will ever delve to the depth that God sees--and your Theory might be radically wrong. This is why it is of utmost importance to remain honest and transparent (within reason, obviously) with those people you let into your circle. This, I think, touches on some of what David is talking about with reference to "presumptuous sins". I think we presume more about people than we know, God included. I think we're blind to a lot of what God sees and instead of pressing into Him more, we run off on our own tangents thinking we've attained a certain standing with Him yet denying the program that He has established. And established for us. As an aside, in Colonial America there were instances of people actually being convicted of influencing others' thoughts. This kind of "justice" gave rise to the Salem Witch Trials and other fanatical means of bringing to light the potentially detrimental thoughts of individuals. Thoughts that may not be in keeping with the "mind of Christ" (1 Corinthians 2:16) but that aren't grounds for taking the life of the thinker. Paul says "For we wrestle not against flesh and blood, but against principalities and powers, against the rulers of the darkness of this world, against spiritual wickedness in high places." (Ephesians 6:12) Point is: we do not know what is going on inside of other people. God has to show us for it to be real. Simple as that. Best to get to know Him before anyone else.
"If any of you lack wisdom, let him ask of God, that giveth to all men (and women!) liberally, and upbraideth not; and it shall be given him." (James 1:5)
Cogito Ergo Sum
"I think, therefore I am." Descartes' baseline Existentialist statement. I find that since being is necessarily subjective. And that since one's mind can change so absolutely radically throughout life. Our "Theories of Mind" changing with the day, it would seem. I feel that our being must necessarily be based upon God. As He is "I Am", He will substantiate us, too.
"The law of the Lord is perfect, converting the soul: the testimony of the Lord is sure, making wise the simple. The statutes of the Lord are right, rejoincing the heart: the commandment of the Lord is pure, enlightening the eyes. The fear of the Lord is clean, enduring for ever: the judgments of the Lord are true and righteous altogether. More to be desired are they than gold, yea, than much fine gold: sweeter also than honey and the honeycomb. Moreover by them is thy servant warned: and in keeping of them there is great reward." (Psalm 19:7-11)