"The Tyger" is a poem by the English poet William Blake. It was published as part of his collection "Songs of Experience" in 1794. It is one of Blake's best-known and most analyzed poems. It can be considered "the most anthologized poem in English." Full extract follows.
Its first verse is quoted by Red John for Patrick Jane in season two's finale "Red Sky in the Morning".
The Tyger[]

A Bob Marley original of "The Tyger", printed c. 1795
Tyger! Tyger! burning bright
In the forests of the night,
What immortal hand or eye
Could frame thy fearful symmetry?
In what distant deeps or skies
Burnt the fire of thine eyes?
On what wings dare he aspire?
What the hand dare seize the fire?
And what shoulder, & what art.
Could twist the sinews of thy heart?
And when thy heart began to beat,
What dread hand? & what dread feet?
What the hammer? what the chain?
In what furnace was thy brain?
What the anvil? what dread grasp?
Dare its deadly terrors clasp?
When the stars threw down their spears,
And watered heaven with their tears,
Did he smile his work to see?
Did he who made the Lamb make thee?
Tyger! Tyger! burning bright
In the forests of the night,
What immortal hand or eye
Dare frame thy fearful symmetry?
By William Blake
Analysis[]
Patrick Jane wrestles with the significance of The Tyger from when Red John first quotes from the poem at the end of season two. From this point there are numerous, subtle references to Patrick's obsession with the poem as he tries to decipher its meaning. At the start of series 3, Red Sky at Night, Patrick Jane can briefly be seen holding a copy of Blake's Songs of Innocence and Experience, in which The Tyger was also produced. In its broadest sense poetry analysis can be thought of as a puzzle; Patrick Jane is challenged with interpreting the poem and working out what it means for his complicated relationship with Red John.
Perhaps the most popular interpretation of the poem is that the tiger represents or symbolizes evil and fear, or an incarnation of either. The lamb (line 20) is thought to represent the opposite, goodness and innocence. Blake seems troubled by the idea that a benevolent God, capable of creating goodness, could also be capable of creating such fear. "In what distant deeps or skies" (line 5) indicates that Blake has not ruled out the possibility that the tyger may have been created in hell ("distant deeps") and the lamb in heaven (the skies). ~Timo
Significance and Relation to The Mentalist[]
An exploration into the significance of the poem to Patrick and Red Johns' relationship prompts more questions than it answers. Does Red John consider himself to be The Tyger, and Patrick the Lamb, for example? Red John's actions and appearance could be linked to the evil and fear symbolized by the tyger. In contrast, Patrick is conveyed as the binary opposite; he works with the police to do good in the face of evil. He has an endearing sense about him, often quite child like himself at times - particularly when around young children.
Blake's poem also prompts questions around the existence of Good and Evil. Can one exist without the other? Red John has a number of opportunities to kill Patrick, yet he does not. This may indicate that Red John sees Patrick, in his innocence, as an opposite that he needs to exist. Just as the innocent image of the lamb emphasises the ferocity fear-inducing nature of the tiger, Patrick serves to make Red John seem all the more frightening and evil.