The question as to whether or not ghosts exist has probably crossed all of our minds at least once. The Biblical answer depends on your definition. The Bible says a lot about spirits of various kinds. One excellent example is found in 1 Samuel 28. It's such a good example because it links back to several other scriptural references.
First, let's read the full passage: “So he said to her, “What is his form?” And she said, “An old man is coming up, and he is covered with a mantle.” And Saul perceived that it was Samuel, and he stooped with his face to the ground and bowed down. Now Samuel said to Saul, “Why have you disturbed me by bringing me up?” And Saul answered, “I am deeply distressed; for the Philistines make war against me, and God has departed from me and does not answer me anymore, neither by prophets nor by dreams. Therefore I have called you, that you may reveal to me what I should do.” Then Samuel said: “So why do you ask me, seeing the LORD has departed from you and has become your enemy? And the LORD has done for Himself as He spoke by me. For the LORD has torn the kingdom out of your hand and given it to your neighbor, David. Because you did not obey the voice of the LORD nor execute His fierce wrath upon Amalek, therefore the LORD has done this thing to you this day. Moreover the LORD will also deliver Israel with you into the hand of the Philistines. And tomorrow you and your sons will be with me. The LORD will also deliver the army of Israel into the hand of the Philistines.” 1 Samuel 28:14-19 NKJV
I'm not going to comment on Samuel's appearance. Let's first look at what Samuel said when he was summoned: "Now Samuel said to Saul, “Why have you disturbed me by bringing me up?” From this, we see that whatever you believe about the dead, whether you believe they are unconscious, or alive in another realm, is ultimately irrelevant. We can see that Samuel, at least, wasn't happy about being disturbed. Therefore, even without the moral aspect, love for God's people ought to be enough to convince us not to seek the dead in Christ.
Saul's response is “I am deeply distressed; for the Philistines make war against me, and God has departed from me and does not answer me anymore, neither by prophets nor by dreams. Therefore I have called you, that you may reveal to me what I should do.” In a moment, we'll see Samuel's response to Saul, but allow me to relate this to Isaiah 8:19-20. It says “And when they say to you, “Seek those who are mediums and wizards, who whisper and mutter,” should not a people seek their God? Should they seek the dead on behalf of the living? To the law and to the testimony! If they do not speak according to this word, it is because there is no light in them.”
Isaiah's words are clear: one should never seek advice from the dead. The dead are no longer part of this world, and should be left where they are. In fact, Isaiah says "to the law and to the testimony", even going as far as to say anyone who says otherwise has no light in them. The law says not to consult with mediums, even prescribing death as a punishment for them (Leviticus 19:31: 20:6, 27; Deuteronomy 18:11-12). We actually know from this same passage that Saul knew about, and enforced this law (1 Samuel 28:9).
Samuel's response to Saul's reasoning is "So why do you ask me, seeing the LORD has departed from you and has become your enemy? And the LORD has done for Himself as He spoke by me." This highlights the authority of scripture. Too many people today are quick to pray or to ask, but too slow to read. For example, Mormons teach that we should "pray to know that the Book of Mormon is true". Why should we pray about it when we have the scriptures? In Acts 17:11, the Bereans studied the scriptures to ensure that Paul was preaching the truth. People also ask their favourite preachers if God will let them into Heaven if (insert their favourite sin here). The response? "Sure! Jesus loves you." But Samuel's response to Saul was effectively "I already told you what God said" (see 1 Samuel 15). Why should Saul have expected the answer to change? And of course, Saul's motive in asking again was to receive an answer he liked. Therefore, God refused to answer at all. James 4:3 tells us that if we "ask amiss", we won't receive.
This appearance from Samuel is conclusive proof that there are some situations in which spirits can be recalled, however there are several important things to consider. First, let me sumarise my points about Saul summoning Samuel, then I will give a few more details that I can't relate directly to this scenario.
• Communication with the dead, either directly or through the help of mediums, is sin. Neither Jew nor Christian can afford to say otherwise.
• While the dead apart from Christ would probably appreciate a break from Hades, if indeed such were possible, the dead in Christ do not wish to be disturbed. Whether you believe they are currently unconscious or in Paradise, they don't want to come back. Therefore, even lacking the moral element, it would still be unloving to summon them.
• Nothing can be gained from spirits that cannot be gained from God. He must be our first and last point of contact, either by prayer, or through the scriptures.
• In the event that a real spirit can be contacted, they will always tell you the same thing.
• Given the nature of this event, it's likely that it is a one off. God allowed the witch to summon Samuel for Saul for the very purpose of this event becoming scripture. There is no evidence that anyone other than Samuel has ever been successfully contacted by a medium.
There is more that the Bible says about ghosts. First, Hebrews 9:27 tells us that we are appointed once to die, then to face judgement. There is no evidence that those on the other side can see, hear, or interact with us any more than we can see, hear, or interact with them. When it comes to mediums, there are three possibilities.
The first, as in Saul's case, is that they genuinely are contacting the dead. Refer back to my bullet points for what happens when this is the case. The second is that the medium is a complete fraud, preying on the gullibility of their audience. The third possibility, however, is far more sinister. In 2 Corinthians 11:14-15, we read “And no wonder! For Satan himself transforms himself into an angel of light. Therefore it is no great thing if his ministers also transform themselves into ministers of righteousness, whose end will be according to their works.” Spirits can, and do, interact with humans, but they are not the spirits of the dead.
The most interesting example I know of is found in Acts 16:16-18. It says “Now it happened, as we went to prayer, that a certain slave girl possessed with a spirit of divination met us, who brought her masters much profit by fortune-telling. This girl followed Paul and us, and cried out, saying, “These men are the servants of the Most High God, who proclaim to us the way of salvation.” And this she did for many days. But Paul, greatly annoyed, turned and said to the spirit, “I command you in the name of Jesus Christ to come out of her.” And he came out that very hour.”
This story is astounding. Luke tells us, first of all, that this spirit is real, and he was genuinely in control of this woman. What's interesting is his testimony. "These men are the servants of the Most High God, who proclaim to us the way of salvation." That's not even a lie. It's the truth. And yet still, Paul was annoyed by this spirit and cast it out. That's how little Paul wanted to do with the spirit world. Even a spirit testifying to the truth of the Gospel, to his own message, and even his own authority from God, was cast out.
Finally, I want to point to Revelation 19:10. John turned to the angel to worship him, but just as Samuel pointed Saul back to God's commands, the angel pointed John back to God, saying "see that you do not do that! I am your fellow servant, and of your brethren who have testimony of Jesus. Worship God! For the testimony of Jesus is the spirit of prophecy." All good spirits will do this. Any focus on them takes away from focus on Jesus.
In summary, ghosts are real. Human beings have an immortal spirit, and in at least three cases (Moses and Elijah came to speak with Jesus in Matthew 17), those spirits have returned to the Earth for a brief time. However, this is not a common occurrence. Ghosts should never be sought out, and when they are, it is far more likely you will find a demon than an actual disembodied spirit. Do not be deceived. Keep your eyes on Jesus.