Friar Lawrence likes to collect various herbs, plants, and flowers that he uses to make all kinds of potions and medicines. Doing so allows him to gain a deep understanding of the natural world, of how the good is so often mixed in with the bad. Everything in nature has a purpose and can conduce to the good if used properly. However, if the bounteous fruits of nature are misused, then the consequences can be unpleasant:

For naught so vile that on the earth doth live / But to the earth some special good doth give. / Nor aught so good but, strained from that fair use / Revolts from true birth, stumbling on abuse. / Virtue itself turns vice, being misapplied, / And vice sometime by action dignified. (Act II, Scene iii).

As we can see from the last lines of the above quotation, Friar Lawrence extends this insight to human beings. Vice can often come out of virtue, and vice versa. There is some interesting foreshadowing going on here. Later on in the play, Friar Lawrence will try to overcome the evil caused by the hatred between the Montagues and the Capulets by marrying Romeo and Juliet.

However, vice also comes out of virtue by way of Friar Lawrence's plan to have Juliet take a sleeping draught designed to give the impression that she's passed away. Romeo and Juliet's marriage and the tragic events that follow illustrate the wisdom of the Friar's insights in his opening soliloquy and monologue.

In Friar Lawrence's opening soliloquy/monologue of Act II, Scene 3, he presents his philosophy of life. The speech is a soliloquy in the first 22 lines as he is alone on stage, and from lines 24-31 it is considered a monologue because Romeo has appeared on stage and the Friar is speaking to him. 

The Friar believes that within the earth "resides" both good and bad elements. One single flower may contain both poison and medicine. The earth is both a womb and a tomb as it produces life and then lets it die. Even the worst poisonous plant provides something positive for the earth. And elements that are essentially good can also wind up doing great harm.

He compares this to humans. Even the worst person may have something positive to contribute. Likewise, a basically good person can commit an evil act. He says,

Within the infant rind of this weak flower
Poison hath residence and medicine power:
For this, being smelt, with that part cheers each
part;
Being tasted, stays all senses with the heart.
Two such opposèd kings encamp them still
In man as well as herbs—grace and rude will;
And where the worser is predominant,
Full soon the canker death eats up that plant.