There are lots of things that a person can get in trouble by possessing: drugs, weapons, and alcohol (if the person is a minor) to name but three.
Possession is defined as actual care, actual custody, actual control or actual management.
For possession to be illegal, it must be voluntary. Possession is voluntary if the possessor possesses the thing for long enough that he could have terminated his control of it.
For possession to be illegal, it must also be knowing. The person possessing the thing must know that he possesses it and know what it is.
If the accused was riding in X’s car, and X was carrying marijuana, the accused did not possess the marijuana (because he didn’t exercise control over it)1.
If the accused was driving his car, and his passenger X was carrying an illegal kinfe, and the accused didn’t know about the illegal knife, the accused didn’t knowingly possess the illegal knife.
If the accused was riding in X’s car and X handed him a paper bag full of cocaine, but the accused didn’t know what was in the bag, he didn’t knowingly possess the cocaine.
If the accused was riding in X’s car and X, seeing flashing lights behind the car, handed his gun to the accused saying “hide my gun,” and if the accused then immediately dropped the gun, the accused didn’t exercise voluntary control over the gun.
The same rules that apply to drugs also hold true for weapons — firearms, illegal knives, brass knuckles, and so forth — and other things that the accused cannot legally possess.
An inferential rebuttal defense is a defense that negates some element of the government’s case. An accused doesn’t have to do anything for an inferential rebuttal defense to apply; the defense attorney could keep his mouth shut throughout the entire trial, and the government would still have to prove beyond a reasonable doubt that none of the inferential rebuttal defenses apply.
(There are two other sorts of defenses under Texas criminal law: defenses that the government must disprove if the accused raises them — entrapment and illegal searches are good examples — and affirmative defenses that the accused must prove by a preponderance of the evidence — for example, insanity.)
In any possession case in Texas, from a gram of marijuana to kilos of cocaine, there are five potential inferential rebuttal defenses
The thing wasn’t what you say it is (it was oregano instead of marijuana).
I didn’t possess (exercise actual control over) the thing.
Even if I possessed (exercised actual control over) the thing, I didn’t know I did.
Even if I possessed it and knew I did, I didn’t know what it was.
Even if I possessed it and knew I did and knew what it was, I didn’t possess it voluntarily.
In a possession with intent to deliver case, there is a sixth inferential rebuttal defense, which, if it is successful, doesn’t negate the entire offense, but reduces its seriousness:
Even if I possessed it and knew I did and knew what it was and possessed it voluntarily, and even if the police didn’t violate the law to get the evidence, I didn’t intend to deliver it.
1 Beware, however, a “law of parties” charge: “Each party to an offense may be charged with the commission of the offense. A person is criminally responsible for an offense committed by the conduct of another if, acting with intent to promote or assist the commission of the offense, he solicits, encourages, directs, aids, or attempts to aid the other person to commit the offense. Mere presence alone will not constitute one a party to an offense.”