Why are three main sections of the novel Hard Times by Charles Dickens entitled Sowing, Reaping, and Garnering?

Charles Dickens's novel Hard Times, which is a social criticism, is divided into three sections. These sections act as thematic titles.

In this section, Dickens introduces his main characters, among whom is Mr. Gradgrind, whose utilitarian fact-philosophy does not allow emotion or imagination to partake in learning or experience. In fact, Gradgrind holds this conviction: 

Facts alone are wanted in life.... Facts: nothing else will ever be of any service.... This is the principle on which I bring up my own children, and this is the principle on which I bring up these children. Stick to Facts....! (Ch.1)

Likewise, just as the factories grind out a product, the educational preparation of Mr. M'Choakumchild is also churning out a product since the children are taught in the same utilitarian manner:

He had worked his stony way...and had taken the bloom off the higher branches of mathematics and physical science, French, German, Latin, and Greek. (I, Ch. 1)

Imagination is discouraged. Even marriage is a practical arrangement. For instance, when Mr. Gradgrind talks with his daughter about marriage with Mr. Bounderby, there is a silence between them as the "deadly statistical clock" sounds hollow. "Father," said Louisa, "do you think I love Mr. Bounderby?" Hearing this, Gradgrind is "discomfited" by such a question about emotion: 

"Well, my child," he returned, "I - really - cannot take upon myself to say." (I, Ch.15)

Through his elimination of imagination and insistence upon cold facts, Mr. Gradgrind calculates everything, even the marriage of his daughter. So, when Louisa remarks, "What does it matter?" it becomes apparent that the seeds of utilitarianism have been sown. 

In the second part of Hard Times, the factories that govern the dreary lives of the workers are the objective correlative of their dehumanization. Lives are a mere drudgery; children's innate play is sacrificed to machinery. In this industrialized environment, there is only the reaping of facts and things without any consideration for feelings. Louisa is in a loveless marriage with an owner of a factory, the much older Bounderby, and her brother Tom is amoral. Their impersonal father, so intent upon teaching his children only "facts," has reaped the product of his philosophy: offspring who have parts of them destroyed.

Further in this second part, Stephen Blackpool, a power-loom operator at Bounderby's factory, informs the other workers of his awareness of their dehumanization as they work for Bounderby: 

Look how we live, ...and look how the mills is awlus a-goin’, and how they never works us no nigher to onny distant object-‘ceptin awlus Death. (II,Ch. 5)

In this factory town, people have been converted into lifeless, loveless mechanical beings. It is a miserable Louisa who later returns to her father and collapses, telling him he has damaged her heart with his lack of warmth, denial of emotion, and insistence upon only facts:

“[W]hat have you done with the garden that should have bloomed once, in this great wilderness here?”(II, 12)

The accumulation of dehumanizing experiences leads to the act of Tom Gradgrind in which he robs a bank and causes Stephen Blackpool to be implicated because Tom has had this poor, uneducated man loiter before Bounderby's bank for a few nights and appear suspicious.

The seeds planted in the first section of the novel, the strict utilitarian Gradgrind's insistence upon facts, and the denial of emotion have led to the amoral behavior of Tom and the despairing collapse of Louisa in the second section. In the third section, then, the characters attempt to "garner," or find, what they have missed in their lives so that they may better their existence, using new emotional resources to stabilize themselves. What has been missed in the "reaping" are the feelings and imagination that Sissy, who has lived with her circus-performer father, finally brings to the Gradgrinds as she moves in with them.