I love words. It is no surprise to me that J. K. Rowling studied modern languages; her books resonate with words that have their roots in other languages and so many of the names in her books ‘play’ on words, a literary device that definitely appeals to the linguist in me.
One of those literary devices is the ‘pensieve’, an imaginary magical repository for memories. 
The idea behind this imaginary device is that if your mind becomes over-crowded with memories, you can ‘dump’ them in the pensieve and sift through them at a later date. The word itself is clearly a play on the word ‘pensive’ (which means ‘thoughtful, meditative, engaged in serious thought’), a word which has its roots in French (penser, to think) and Latin (pensare, to weigh or consider carefully), but the spelling ‘pensieve’ also brings to mind a ‘sieve’, a device often resembling a screen that can be used to sift through something. Sieves are often used to eliminate undesired objects or items, such as when an archaeologist uses one to sift through sand or dirt in search of fossils or other relics. The pensieve seems to be a way to prevent people’s often sieve-like memories from losing important information.
Ps 139 starts with the realisation ‘You have searched me, Lord, and You know me’ (Ps 139:1) and concludes with David’s prayer “Search me, God, and know my heart; test me and know my anxious thoughts. See if there is any offensive way in me, and lead me in the way everlasting.” (Ps 139:23-24)
We all need some kind of ‘pensieve’, some way of sifting our thoughts and retaining only that which is good, profitable, true, right, noble, praiseworthy or admirable. We need some external method of discerning our own motives and thoughts, for as Jeremiah tells us, the heart is desperately deceitful (Jer 17:9). That’s why we need to pray as David did, ‘search me and know me’, or, as the Message puts it, ‘investigate my life, God; find out everything about me.’
Without the insight brought by God’s Holy Spirit and the piercing revelation of His Word (that word which is like a double-edged sword or sharp scalpel, Hebrews 4:12), we are capable of self-delusion and misunderstanding on a scale that is almost bewildering. We desperately need God’s revelation to uncover the duplicity of our hearts and bring us to that place of honest repentance. David committed murder and adultery and apparently had no qualms of conscience until confronted by the prophet Nathan, whose story pierced his defences. Only then did he say ‘I know my transgressions’ (Ps 51:3).
May we be honest enough and willing enough to let God search us and know us and lead us in the way everlasting.