Jenny writes,

This is my math-and-philosophy tattoo. The purple bit is the math symbol for “to define”, used in proofs to set [variable on left] equal to [stuff on right]. The eye is both a rebus for “I” and a symbol for “to observe”, and it’s in front of the definition symbol to indicate that observation comes first, definition follows. Or as Sherlock Holmes put it, “It is a capital mistake to theorize in advance of the facts.” So it means “I observe and then define”, and “Data before decisions”, and “I decide for myself (based on my own observation)”, and therefore implies “Think for yourself.”