The writer of Hebrews is refering to this precept found in Scripture....
Psa 119:97 MEM. O how love I thy law! it [is] my meditation all the day.
Psa 119:98 Thou through thy commandments hast made me wiser than mine enemies: for they [are] ever with me.
Psa 119:99 I have more understanding than all my teachers: for thy testimonies [are] my meditation.
Psa 119:100 I understand more than the ancients, because I keep thy precepts.
Psa 119:101 I have refrained my feet from every evil way, that I might keep thy word.
Psa 119:102 I have not departed from thy judgments: for thou hast taught me.
Psa 119:103 How sweet are thy words unto my taste! [yea, sweeter] than honey to my mouth!
Psa 119:104 Through thy precepts I get understanding: therefore I hate every false way.
Now those who the writer of Hebrews exhorts have this issue...
Hbr 5:11 Of whom we have many things to say, and hard to be uttered, seeing ye are dull of hearing.
Hbr 5:12 ¶ For when for the time ye ought to be teachers, ye have need that one teach you again which [be] the first principles of the oracles of God; and are become such as have need of milk, and not of strong meat.
Hbr 5:13 For every one that useth milk [is] unskilful in the word of righteousness: for he is a babe.
Hbr 5:14 But strong meat belongeth to them that are of full age, [even] those who by reason of use have their senses exercised to discern both good and evil.
Compare the writer of Psalm 119's testimony and what he was enabled to do...with the lack that is recognized in the lives of the Hebrew church...
Focus on what empowers one to discern good from evil...what are the consequences of a life that does not seek to grow in the wisdom of the word of God...
In Christ
Jeff