Sculpted by: Israel Skelton
Release: 2014
Edition: Limited
Used material(s): Cast in copper and gold-plated pewter
One of: 1000

Signed: Chris Ryall

First appearance: Keys to the Kingdom #4
Use the Philosophoscope
Philosopscope telescope

More information:
(possible spoilers)

The Philosophoscope Key was used by Dodge to spy on Keyhouse Manor. He uses it to supposedly find out who an “Untrustworthy Ally” is, and ends up spying on Rufus (who he now believes to be said untrustworthy ally) and Sam Lesser (who is the real Untrustworthy Ally, but cannot be seen by the Philosophoscope while he is a ghost).

The settings that are seen on the Philosophoscope are as follows:

“Beste Teacher” (Best Teacher)
“Truest Love”
“Usefullest Soulle” (Most Useful Soul/Person)
“Untrustworthie Ally” (Untrustworthy Ally)
“Grave Hazzyrd” (Grave Hazard)
While there may be more settings, these appear to be the only ones. When the user looks through the scope, it shows the person who best fits the desired category.

Guide to the known keys entry:
“…Hannes asked me to come with him, in his quiet inflected English. I told him I liked it better when I thought he was mute because at least then he couldn’t tell me lies. But I took his hand and went with him. He walked me to the Philosophoscope in the tower. Once I realized where we were going, johnnie, I didn’t want to go, and couldn’t help myself. Once I realized what he was going to show me. He put the Key in, and shifted the lever to Truest Love and asked me to look. Oh Johnnie. How I have lied to myself for months. He knew what I would see somehow even thought I didn’t…that I would see him…”

Source: Locke & Key Wikia